commit 4076440ff3910d0e9895b217376ffd23ae3b6bdf
Author: Sander Hautvast <shautvast@gmail.com>
Date:   Fri Feb 12 17:42:15 2021 +0100

    supports vectors with functions, methods, properties

diff --git a/.DS_Store b/.DS_Store
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diff --git a/.gitignore b/.gitignore
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+++ b/.gitignore
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+.idea/
+*.iml
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diff --git a/README.md b/README.md
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+++ b/README.md
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+**MatRepl**
+* is a Matrix
+* and a repl: Read–Eval–Print Loop, where Print is doing operations on vectors and matrices in a graphic environment
+
+<img src="screenshot1.png"></img>
+  
+The repl has the following syntax (It's work in progress, new capabilities will be added)
+* simple arithmetic expressions: 
+  ** add, subtract, divide, multiply
+  ** variable declaration eg: a= ...
+  ** vector(x0,y0,x,y) adds a vector
+  ** remove(x) removes bindings (when it's an object (eg vector), removes it from the matrix)
+  ** method calls:
+  *** a = vector(0,0,12,1)
+  *** a.type()
+  *** > vector
+  ** property lookup
+  *** a.x
+  *** 12
+  
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diff --git a/screenshot-1.png b/screenshot-1.png
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diff --git a/src/app.css b/src/app.css
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+++ b/src/app.css
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+body {
+    font: 13px Arial, sans-serif;
+    background: black;
+    overflow: hidden;
+}
+
+.background {
+    position: absolute;
+    left: 0;
+    top: 0;
+    width: 100%;
+    height: 100%;
+}
+
+svg {
+    position: absolute;
+    width: 100%;
+    height: 100%;
+    z-index: -1;
+}
+
+.grid {
+    fill: none;
+    stroke: #4682b4;
+    stroke-width: 3;
+}
+
+.bg-grid {
+    fill: none;
+    stroke: gray;
+    stroke-width: 0.5;
+}
+
+#console {
+    padding: 5px;
+    color: greenyellow;
+    background: black;
+    position: absolute;
+    right: 10px;
+    bottom: 0;
+    width: 40%;
+    height: 10em;
+    border: 2px solid darkgray;
+    border-radius: 10px;
+    z-index: 10;
+}
+
+.axis {
+    stroke-width: 1.5;
+    stroke: lightpink;
+}
+
+.vector {
+    stroke-width: 2.5;
+    stroke: yellow;
+    stroke-linecap: round;
+}
+
+#prompt{
+    position: absolute;
+    bottom: 0;
+}
+
+#command_input{
+    border: none transparent;
+    background: black;
+    color: greenyellow;
+    outline: none;
+    width: 39vw;
+}
+
+#command_history{
+    font-size: 12px;
+    color: greenyellow;
+    position: absolute;
+    bottom: 1.5em;
+    max-height: 9em;
+    width: 100%;
+    overflow-y: visible;
+    overflow-x: hidden;
+}
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diff --git a/src/console.js b/src/console.js
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index 0000000..1b38c84
--- /dev/null
+++ b/src/console.js
@@ -0,0 +1,183 @@
+/**
+ * handles user input from the console div
+ */
+(function () {
+        const state = {};
+        const command_input_element = document.getElementById('command_input');
+        const command_history_element = document.getElementById('command_history');
+        const bottom = document.getElementById('bottom');
+        command_input_element.value = '';
+        let command_history = [];
+        let command_history_index = 0;
+
+        command_input_element.onkeyup = function handle_key_input(event) {
+            if (event.key === 'ArrowUp') {
+                if (command_history_index > -1) {
+                    command_input_element.value = command_history[command_history_index];
+                    if (command_history_index > 0) {
+                        command_history_index -= 1;
+                    }
+                }
+            }
+            if (event.key === 'ArrowDown') {
+                if (command_history_index < command_history.length - 1) {
+                    command_history_index += 1;
+                    command_input_element.value = command_history[command_history_index];
+                } else {
+                    command_input_element.value = '';
+                }
+            }
+            if (event.key === 'Enter') {
+                let command = command_input_element.value;
+                command_history_element.innerText += command + "\n";
+                command_input_element.value = '';
+                command_history_index = command_history.length;
+                let tokens = scan(command);
+                let statement = parse(tokens);
+                let result;
+                try {
+                    result = visit_expression(statement);
+                    if (result.description) {
+                        result = result.description;
+                    }
+                } catch (e) {
+                    result = e.message;
+                }
+                command_history_element.innerText += result + "\n";
+                command_history.push(command);
+                command_history_element.scrollTo(0, command_history_element.scrollHeight);
+            }
+        };
+
+        let visit_expression = function (expr) {
+            switch (expr.type) {
+                case 'declaration':
+                    let value = visit_expression(expr.initializer);
+                    let existing_value = state[expr.var_name.value];
+                    if (existing_value) {
+                        if (existing_value.type === 'vector') {
+                            remove_vector(existing_value.object); // remove from screen
+                        }
+                    }
+                    value.binding = expr.var_name.value;
+                    state[expr.var_name.value] = value;
+                    let description = state[expr.var_name.value].description;
+                    if (!description) {
+                        description = state[expr.var_name.value]; //questionable. use toString instead of message?
+                    }
+                    return {description: expr.var_name.value + ':' + description};
+                case 'group':
+                    return visit_expression(expr.expression);
+                case 'unary':
+                    let right_operand = visit_expression(expr.right);
+                    if (expr.operator === token_types.MINUS) {
+                        return -right_operand;
+                    } else if (expr.operator === token_types.NOT) {
+                        return !right_operand;
+                    } else {
+                        throw {message: 'illegal unary operator'};
+                    }
+                case 'binary':
+                    let left = visit_expression(expr.left);
+                    let right = visit_expression(expr.right);
+                    switch (expr.operator) {
+                        case token_types.MINUS:
+                            return left - right;
+                        case token_types.PLUS:
+                            return addition(left, right);
+                        case token_types.STAR:
+                            return multiplication(left, right);
+                        case token_types.SLASH:
+                            return left / right;
+                        case token_types.DOT:
+                            return method_call(left, expr.right);
+                    }
+                    throw {message: 'illegal binary operator'}
+                case 'identifier': {
+                    if (state[expr.name]) {
+                        return state[expr.name];
+                    } else {
+                        break;
+                    }
+                }
+                case 'literal':
+                    return expr.value;
+                case 'call':
+                    return call(expr.name, expr.arguments);
+            }
+        }
+
+        const call = function (function_name, argument_exprs) {
+            let arguments = [];
+            for (let i = 0; i < argument_exprs.length; i++) {
+                arguments.push(visit_expression(argument_exprs[i]));
+            }
+            if (functions[function_name]) {
+                return functions[function_name](arguments);
+            } else {
+                let arg_list = '';
+                for (let i = 0; i < argument_exprs.length; i++) {
+                    if (i > 0) {
+                        arg_list += ',';
+                    }
+                    arg_list += argument_exprs[i].value_type;
+                }
+                return 'unimplemented: ' + function_name + '(' + arg_list + ')';
+            }
+        }
+
+        const method_call = function (object_wrapper, method_or_property) {
+            if (object_wrapper) {
+                if (method_or_property.type === 'call') { // method
+                    if (typeof object_wrapper.object[method_or_property.name] !== 'function') {
+                        throw {message: `method ${method_or_property.name} not found on ${object_wrapper.type}`};
+                    }
+                    return object_wrapper.object[method_or_property.name].apply(object_wrapper, method_or_property.arguments);
+
+                } else { // property
+                    if (!object_wrapper.object.hasOwnProperty(method_or_property.name)){
+                        throw {message: `property ${method_or_property.name} not found on ${object_wrapper.type}`};
+                    }
+                    return object_wrapper.object[method_or_property.name];
+                }
+            } else {
+                throw {message: `not found: ${object_wrapper}`};
+            }
+        }
+
+        const functions = {
+            help: () => help(),
+            vector: (args) => add_vector({x0: args[0], y0: args[1], x: args[2], y: args[3]}),
+            remove: (args) => {
+                if (args[0].hasOwnProperty('binding')) {
+                    delete this.state[args[0].binding];
+                    return remove_vector(args[0].object); // by binding value
+                } else {
+                    return remove_vector(args[0]); // by index (@...)
+                }
+
+            },
+        }
+
+        const help = function () {
+            return {message: 'vector(x0, y0, x, y): draws a vector from x0,y0 to x,y'}
+        }
+
+        const multiplication = function (left, right) {
+            if (left.object && left.type === 'vector' && !right.object) {
+                return left.object.multiply(right);
+            }
+            if (right.object && right.type === 'vector' && !left.object) {
+                return right.object.multiply(left);
+            }
+            return left * right;
+        }
+
+        const addition = function (left, right) {
+            if (left.object && left.type === 'vector' && right.object && right.type === 'vector') {
+                return left.object.add(right.object);
+            }
+            return left + right;
+        }
+    }
+)();
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diff --git a/src/index.html b/src/index.html
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+++ b/src/index.html
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+<!DOCTYPE html>
+<html lang="en">
+<head>
+    <meta charset="UTF-8">
+    <title>Interactive Linear Algebra</title>
+    <link rel="stylesheet" href="app.css">
+</head>
+<body>
+<div id="console">
+    <div id="command_history"><div id="bottom"></div></div>
+    <label id="prompt">&gt;
+        <input id="command_input" type="text" autofocus/>
+    </label>
+</div>
+<script type="application/ecmascript" src="index.js"></script>
+<script type="application/ecmascript" src="console.js"></script>
+<script type="application/ecmascript" src="scanner.js"></script>
+<script type="application/ecmascript" src="parser.js"></script>
+</body>
+</html>
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diff --git a/src/index.js b/src/index.js
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--- /dev/null
+++ b/src/index.js
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+let add_vector,
+    remove_vector;
+
+/**
+ * Main entry. draws the matrix
+ */
+(function () {
+    const SVG_NS = 'http://www.w3.org/2000/svg'; // program needs these to create svg elements
+    let grid_size = 100; // this is the nr of pixels for the basis vector (1,0) (0,1)
+    let half_grid_size = grid_size >> 1; // used to position the grid lines
+    let vectors = []; // collection of added vectors
+    let moving_vector; // user can move vector arrows. when moving, this refers to the arrow
+    let width = window.innerWidth, height = window.innerHeight;
+    let origin_x = Math.floor((width / grid_size) / 2) * grid_size + half_grid_size,
+        origin_y = Math.floor((height / grid_size) / 2) * grid_size + half_grid_size;
+    /**
+     * Creates an svg element
+     * @param element_type path,g, etc
+     * @returns SVG element
+     */
+    const create = function (element_type) {
+        return document.createElementNS(SVG_NS, element_type);
+    }
+
+    /**
+     * creates the d attribute string
+     * @param x0 start_x
+     * @param y0 start_y
+     * @param x1 end_x
+     * @param y1 end y
+     * @returns {string} to put in an SVG path
+     */
+    const calculate_d = function (x0, y0, x1, y1) {
+        return "M" + x0 + " " + y0 + " L" + x1 + " " + y1;
+    }
+
+    /**
+     * creates a SVG line (path)
+     * @param x0 start_x
+     * @param y0 start_y
+     * @param x1 end_x
+     * @param y1 end_y
+     * @param css_class the css class to make up the element
+     * @returns an SVG path element
+     */
+    const create_line = function (x0, y0, x1, y1, css_class) {
+        let path = create('path');
+        path.setAttribute('d', calculate_d(x0, y0, x1, y1));
+        path.setAttribute('class', css_class);
+        return path;
+    }
+
+    /**
+     * creates the arrow path element
+     * @param id attribute
+     * @param x0 start_x
+     * @param y0 start_y
+     * @param x1 end_x
+     * @param y1 end_y
+     * @param css_class class attribute
+     * @returns {SVGPathElement}
+     */
+    const arrow = function (id, x0, y0, x1, y1, css_class) {
+        let path = create('path');
+
+        path.setAttribute('d', calculate_d(x0, y0, x1, y1));
+        path.id = id;
+        path.setAttribute('class', css_class);
+        path.setAttribute('marker-end', 'url(#arrow)');
+        return path;
+    }
+
+    /**
+     * Draws the background grid of the space
+     * @param css_class class for the lines that are 'multiples of the basis vector'
+     * @param bg_css_class class for in between lines
+     * @returns {SVGGElement}
+     */
+    const create_grid = function (css_class, bg_css_class) {
+        const group = create('g');
+        group.setAttribute('id', 'grid');
+        const horizontal = create('g');
+        horizontal.setAttribute('id', 'horizontal');
+        for (let y = 0; y < height; y += grid_size) {
+            horizontal.appendChild(create_line(0, y + half_grid_size, width, y + half_grid_size, css_class));
+            horizontal.appendChild(create_line(0, y, width, y, bg_css_class));
+        }
+        group.appendChild(horizontal);
+        const vertical = create('g');
+        vertical.setAttribute('id', 'vertical');
+        for (let x = 0; x < width; x += grid_size) {
+            vertical.appendChild(create_line(x + half_grid_size, 0, x + half_grid_size, height, css_class));
+            vertical.appendChild(create_line(x, 0, x, height, bg_css_class));
+        }
+        group.appendChild(vertical);
+        return group;
+    }
+
+    /**
+     * removes child from element by id if found
+     * @param element
+     * @param child_id id to remove
+     */
+    const remove_child = function (element, child_id) {
+        let node = element.firstChild;
+        while (node && child_id !== node.id) {
+            node = node.nextSibling;
+        }
+        if (node) {
+            element.removeChild(node);
+        }
+    }
+
+    /**
+     * removes the grid from the DOM and adds an updated one.
+     */
+    const redraw_grid = function () {
+        remove_child(svg, "grid");
+        svg.appendChild(create_grid('grid', 'bg-grid'));
+        svg.appendChild(create_axes());
+    }
+
+    /**
+     * Adds a vector to the set.
+     * @param vector
+     */
+    add_vector = function (vector) {
+        vector.id = vectors.length;
+        vectors.push(vector);
+        redraw();
+        vector.add = (other) => add_vector({
+            x0: vector.x0 + other.x0,
+            y0: vector.x0 + other.x0,
+            x: vector.x + other.x,
+            y: vector.y + other.y
+        });
+        vector.multiply = (scalar) => add_vector({
+            x0: vector.x0 * scalar,
+            y0: vector.y0 * scalar,
+            x: vector.x * scalar,
+            y: vector.y * scalar
+        });
+        vector.is_vector = true;
+        vector.type = () => 'vector';
+        return { //object_wrapper
+            type: 'vector',
+            object: vector,
+            description: `vector@${vector.id}{x0:${vector.x0},y0:${vector.y0} x:${vector.x},y:${vector.y}}`,
+        };
+
+    }
+
+    remove_vector = function (vector_or_index) {
+        let index;
+        if (vector_or_index.is_vector) {
+            for (let i = 0; i < vectors.length; i++) {
+                if (vectors[i].id === vector_or_index.id) {
+                    index = i;
+                    break;
+                }
+            }
+        } else {
+            index = vector_or_index;
+        }
+
+        if (!vectors[index]) {
+            throw {message: `vector@${index} not found`};
+        }
+
+        vectors.splice(index, 1);
+        redraw();
+        return {description: `vector@${index} removed`};
+    }
+
+    /**
+     * The moving operation. Called by onmousemove on the svg ('canvas')
+     * @param event
+     */
+    const move = function (event) {
+        if (moving_vector) {
+            let current_x = event.clientX;
+            let current_y = event.clientY;
+            vectors[moving_vector.id].x = (current_x - origin_x) / grid_size;
+            vectors[moving_vector.id].y = (origin_y - current_y) / grid_size;
+            moving_vector.setAttribute('d', calculate_d(origin_x, origin_y, current_x, current_y));
+        }
+    }
+
+    /**
+     * Draws all the vectors.
+     *
+     * vector {
+     *     x0,y0 origin
+     *     x,y coordinates
+     * }
+     */
+    const draw_vectors = function () {
+        const vector_group = create("g");
+        vector_group.id = 'vectors';
+
+        for (let i = 0; i < vectors.length; i++) {
+            let vector_arrow = arrow(vectors[i].id,
+                origin_x + vectors[i].x0 * grid_size,
+                origin_y - vectors[i].y0 * grid_size,
+                origin_x + vectors[i].x * grid_size,
+                origin_y - vectors[i].y * grid_size,
+                'vector');
+            vector_arrow.onmousedown = function start_moving_vector(event) {
+                moving_vector = event.target;
+            };
+            vector_group.appendChild(vector_arrow);
+        }
+        svg.appendChild(vector_group);
+    }
+
+    /**
+     * Removes all vectors in the svg and calls draw_vectors to draw updated versions.
+     */
+    const redraw_vectors = function () {
+        remove_child(svg, 'vectors');
+        draw_vectors();
+    }
+
+    /**
+     * (re)draws all
+     */
+    const redraw = function () {
+        redraw_grid();
+        redraw_vectors();
+    }
+
+    const create_axes = function () {
+        let axes_group = create('g');
+        let x = create_line(0, origin_y, width, origin_y, 'axis');
+        x.id = 'x-axis';
+        axes_group.appendChild(x);
+        let y = create_line(origin_x, 0, origin_x, height, 'axis');
+        y.id = 'y-axis';
+        axes_group.appendChild(y);
+        return axes_group;
+    }
+
+    /**
+     * setup the arrow head for the vector
+     * @returns {SVGDefsElement}
+     */
+    function create_defs() {
+        let defs = create('defs');
+        let marker = create('marker');
+        marker.id = 'arrow';
+        marker.setAttribute('orient', 'auto');
+        marker.setAttribute('viewBox', '0 0 10 10');
+        marker.setAttribute('markerWidth', '3');
+        marker.setAttribute('markerHeight', '4');
+        marker.setAttribute('markerUnits', 'strokeWidth');
+        marker.setAttribute('refX', '6');
+        marker.setAttribute('refY', '5');
+        let polyline = create('polyline');
+        polyline.setAttribute('points', '0,0 10,5 0,10 1,5');
+        polyline.setAttribute('fill', 'yellow');
+        marker.appendChild(polyline);
+        defs.appendChild(marker);
+        return defs;
+    }
+
+    /**
+     * Creates the SVG
+     * @returns {SVGElement}
+     */
+    const create_svg = function () {
+        let svg = create('svg');
+
+        svg.onmousemove = move;
+        svg.onmouseup = function stop_moving_vector() {
+            moving_vector = undefined;
+        };
+
+        let defs = create_defs();
+        svg.appendChild(defs);
+        return svg;
+    }
+
+    document.body.onresize = function recalculate_window_dimensions() {
+        width = window.innerWidth;
+        height = window.innerHeight;
+        origin_x = Math.floor((width / grid_size) / 2) * grid_size + half_grid_size;
+        origin_y = Math.floor((height / grid_size) / 2) * grid_size + half_grid_size;
+        redraw();
+    }
+
+    const svg = create_svg();
+    document.body.appendChild(svg);
+
+    svg.appendChild(create_grid('grid', 'bg-grid'));
+    svg.appendChild(create_axes());
+})
+();
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diff --git a/src/parser.js b/src/parser.js
new file mode 100644
index 0000000..a3f8d74
--- /dev/null
+++ b/src/parser.js
@@ -0,0 +1,185 @@
+const parse = function (tokens) {
+    let token_index = 0;
+
+    return statement();
+
+    function statement() {
+        if (check(token_types.IDENTIFIER, token_index) && check(token_types.EQUALS, token_index + 1)) {
+            let var_name = current_token();
+            advance();
+            advance();
+            return {type: 'declaration', var_name: var_name, initializer: expression()};
+        } else {
+            return expression();
+        }
+    }
+
+    function expression() {
+        return equality();
+    }
+
+    function equality() {
+        let expr = comparison()
+
+        while (match([token_types.EQUALS_EQUALS, token_types.NOT_EQUALS])) {
+            let operator = previous_token();
+            let right = unary();
+            expr = {type: 'binary', left: expr, operator: operator, right: right};
+        }
+
+        return expr;
+    }
+
+    function comparison() {
+        let expr = addition();
+
+        while (match([token_types.LESS, token_types.LESS_OR_EQUAL, token_types.GREATER, token_types.GREATER_OR_EQUAL])) {
+            let operator = previous_token();
+            let right = addition();
+            expr = {type: 'binary', left: expr, operator: operator, right: right};
+        }
+
+        return expr;
+    }
+
+    function addition() {
+        let expr = multiplication();
+
+        while (match([token_types.MINUS, token_types.PLUS])) {
+            let operator = previous_token();
+            let right = multiplication();
+            expr = {type: 'binary', left: expr, operator: operator, right: right};
+        }
+
+        return expr;
+    }
+
+    function multiplication() {
+        let expr = unary();
+
+        while (match([token_types.SLASH, token_types.STAR, token_types.DOT])) {
+            let operator = previous_token();
+            let right = unary();
+            expr = {type: 'binary', left: expr, operator: operator, right: right};
+        }
+
+        return expr;
+    }
+
+    function unary() {
+        if (match([token_types.NOT, token_types.MINUS])) {
+            let operator = previous_token();
+            let right = unary();
+            return {type: 'unary', operator: operator, right: right};
+        } else {
+            return call();
+        }
+    }
+
+    function call() {
+        let expr = primary();
+
+        while (true) {
+            if (match([token_types.LEFT_PAREN])) {
+                expr = finish_call(expr.name);
+            } else {
+                break;
+            }
+        }
+
+        return expr;
+    }
+
+    function finish_call(callee) {
+        let arguments = [];
+        if (!check(token_types.RIGHT_PAREN, token_index)) {
+            do {
+                arguments.push(expression());
+            } while (match([token_types.COMMA]));
+        }
+        if (!match([token_types.RIGHT_PAREN])) {
+            throw {message: "Expect ')' after arguments."};
+        }
+
+        return {type: 'call', name: callee, arguments: arguments};
+    }
+
+
+    function primary() {
+        if (match([token_types.NUMERIC, token_types.STRING])) {
+            return {type: 'literal', value: previous_token().value, value_type: previous_token().type};
+        } else if (match([token_types.LEFT_PAREN])) {
+            let expr = expression();
+            if (expr && match([token_types.RIGHT_PAREN])) {
+                return {type: 'group', expression: expr};
+            } else {
+                throw {message: 'expected expression or )'};
+            }
+        } else if (check(token_types.IDENTIFIER, token_index)) {
+            let identifier = {type: 'identifier', name: current_token().value};
+            advance();
+            return identifier;
+        }
+    }
+
+    /**
+     * matches token against array of tokens to check for equality (matching type)
+     * @param tokens_to_match array of tokens
+     * @returns {boolean}
+     */
+    function match(tokens_to_match) {
+        for (let i = 0; i < tokens_to_match.length; i++) {
+            if (are_same(tokens_to_match[i], current_token())) {
+                advance()
+                return true;
+            }
+        }
+        return false;
+    }
+
+    /**
+     * Checks if token at position index matches the given
+     * @param token_to_check expected token type
+     * @param index of token to check
+     * @returns {boolean}
+     */
+    function check(token_to_check, index) {
+        let token = tokens[index];
+        if (!token) {
+            return false;
+        }
+        return are_same(token_to_check, token);
+
+    }
+
+    /**
+     * checks if 2 tokens have same type
+     * @param token_1
+     * @param token_2
+     * @returns {boolean}
+     */
+    function are_same(token_1, token_2) {
+        if (is_at_end()) {
+            return false;
+        } else {
+            return token_1.type === token_2.type;
+        }
+
+    }
+
+    function is_at_end() {
+        return token_index >= tokens.length;
+    }
+
+    function advance() {
+        token_index += 1;
+    }
+
+    function previous_token() {
+        return tokens[token_index - 1];
+    }
+
+    function current_token() {
+        return tokens[token_index];
+    }
+}
\ No newline at end of file
diff --git a/src/scanner.js b/src/scanner.js
new file mode 100644
index 0000000..92be530
--- /dev/null
+++ b/src/scanner.js
@@ -0,0 +1,181 @@
+/**
+ * Creates an array of tokens from a line of input.
+ *
+ * @param command: string
+ * @returns {token_type[]}
+ */
+const scan = function(command) {
+    let current_index = 0, // current index of char to look at in the command string
+        word_start_index = 0, // marker for start of a literal or identifier
+        tokens = [];
+
+    while (!is_at_end()) {
+        word_start_index = current_index;
+        let token = scan_token();
+        if (token) { // undefined mostly means whitespace
+            tokens.push(token);
+        }
+    }
+    return tokens;
+
+    function scan_token() {
+        let next_char = advance();
+        switch (next_char) {
+            case '(':
+                return token_types.LEFT_PAREN;
+            case ')':
+                return token_types.RIGHT_PAREN;
+            case '[':
+                return token_types.LEFT_BRACKET;
+            case ']':
+                return token_types.RIGHT_BRACKET;
+            case ',':
+                return token_types.COMMA;
+            case '.':
+                return token_types.DOT;
+            case '-':
+                return token_types.MINUS;
+            case '+':
+                return token_types.PLUS;
+            case '*':
+                return token_types.STAR;
+            case '/':
+                return token_types.SLASH;
+            case '>':
+                if (expect('=')) {
+                    return token_types.GREATER_OR_EQUAL;
+                } else {
+                    return token_types.GREATER;
+                }
+            case '<':
+                if (expect('=')) {
+                    return token_types.LESS_OR_EQUAL;
+                } else {
+                    return token_types.LESS;
+                }
+            case '!':
+                if (expect('=')) {
+                    return token_types.NOT_EQUALS;
+                } else {
+                    return token_types.NOT;
+                }
+            case '=':
+                if (expect('=')) {
+                    return token_types.EQUALS_EQUALS;
+                } else {
+                    return token_types.EQUALS;
+                }
+            case '\'':
+                return string('\'');
+            case '\"':
+                return string('\"');
+        }
+        if (is_digit(next_char)) {
+            let token = Object.assign({}, token_types.NUMERIC);
+            token.value = parse_number();
+            return token;
+        } else {
+            if (is_alpha_or_underscore(next_char)) {
+                let token = Object.assign({}, token_types.IDENTIFIER);
+                token.value = parse_identifier();
+                return token;
+            }
+        }
+    }
+
+    function expect(expected_char) {
+        if (is_at_end()) {
+            return false;
+        }
+        if (current_char() === expected_char) {
+            advance();
+            return true;
+        } else {
+            return false;
+        }
+    }
+
+    function advance() {
+        if (current_index < command.length) {
+            current_index += 1;
+        }
+        return command[current_index - 1];
+    }
+
+    function is_at_end() {
+        return current_index >= command.length;
+    }
+
+    function current_char() {
+        return command[current_index];
+    }
+
+    function is_digit(char) {
+        return char >= '0' && char <= '9';
+    }
+
+    function is_part_of_number(char) {
+        return is_digit(char) || char === '.'; // no scientific notation for now
+    }
+
+    function parse_number() {
+        while (is_part_of_number(current_char())) {
+            advance();
+        }
+        let number_string = command.substring(word_start_index, current_index);
+        return Number.parseFloat(number_string);
+    }
+
+    function is_alpha_or_underscore(char) {
+        return (char >= 'a' && char <= 'z') || (char >= 'A' && char <= 'Z') || char === '_';
+    }
+
+    function is_alphanumeric_or_underscore(char) {
+        return (char >= 'a' && char <= 'z') || (char >= 'A' && char <= 'Z') || is_digit(char) || char === '_';
+    }
+
+    function parse_identifier() {
+        while (is_alphanumeric_or_underscore(current_char())) {
+            advance();
+        }
+        return command.substring(word_start_index, current_index);
+    }
+
+    function string(quote) { // as of yet strings may not unclude escaped quotes that are also the start/end quote
+        while (current_char() !== quote && !is_at_end()) {
+            advance();
+        }
+        if (is_at_end() && current_char() !== quote) {
+            throw {message: 'unterminated string'}
+        } else {
+            let string_token = Object.assign({}, token_types.STRING);
+            string_token.value = command.substring(word_start_index + 1, current_index);
+            advance();
+            return string_token;
+        }
+    }
+};
+
+const token_types = {
+    LEFT_PAREN: {type: 'left_paren'},
+    RIGHT_PAREN: {type: 'right_paren'},
+    LEFT_BRACKET: {type: 'left_bracket'},
+    RIGHT_BRACKET: {type: 'right_bracket'},
+    COMMA: {type: 'comma'},
+    DOT: {type: 'dot'},
+    MINUS: {type: 'minus'},
+    PLUS: {type: 'plus'},
+    STAR: {type: 'star'},
+    SLASH: {type: 'slash'},
+    EQUALS: {type: 'equals'},
+    EQUALS_EQUALS: {type: 'equals_equals'},
+    NOT_EQUALS: {type: 'not_equals'},
+    NOT: {type: 'not'},
+    GREATER: {type: 'greater'},
+    GREATER_OR_EQUAL: {type: 'greater_or_equal'},
+    LESS: {type: 'less'},
+    LESS_OR_EQUAL: {type: 'less_or_equal'},
+    NUMERIC: {type: 'number', value: undefined},
+    IDENTIFIER: {type: 'identifier', value: undefined},
+    STRING: {type: 'string', value: undefined}
+};