Add register machine evaluator (5.4 SICP JS)

doc/register-machine-evaluator.md

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+# CS4215 T1 GC for Register Machine
+
+## Overview
+
+This implements a register machine evaluator for Source §1, as described in [<https://sicp.comp.nus.edu.sg/chapters/109#top>](Chapter 5.4) of SICP JS. It adopts the evaluator from Chapter 4.1 into machine instructions.
+
+Register machine code format is found [<https://github.com/source-academy/source-programs/wiki/Register-Machine-Code-Format>](in this wiki).
+
+## Usage
+
+Example:
+
+```js
+set_program_to_run("1 + 3 * 4;");   // This parses the program and loads it into the register machine
+start(m);                           // This runs the register machine
+get_register_contents(m, "val");    // The output is in the register named "val"
+```
+
+## Testing
+
+Test cases are provided.
+
+Install dependencies by running `yarn`.
+
+Run tests by executing `yarn test`.

src/virtual-machines/__tests__/register-machine-count-leaves.test-1.js

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+set_register_contents(cl, "curr_node", pair(pair(1, 2), pair(3, pair(4, 5))));
+start(cl);
+get_register_contents(cl, "num_leaves");
+
+// 5

src/virtual-machines/__tests__/register-machine-count-leaves.test-2.js

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+set_register_contents(cl, "curr_node", 1);
+start(cl);
+get_register_contents(cl, "num_leaves");
+
+// 1

src/virtual-machines/__tests__/register-machine-count-leaves.test-null.js

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+set_register_contents(cl, "curr_node", null);
+start(cl);
+get_register_contents(cl, "num_leaves");
+
+// 0

src/virtual-machines/__tests__/register-machine-explicit-control-evaluator.addition.js

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+set_program_to_run("1 + 1;");
+
+start(m);
+
+get_register_contents(m, "val");
+
+// 2

src/virtual-machines/__tests__/register-machine-explicit-control-evaluator.arithmetic-1.js

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+set_program_to_run("1.4 / 2.3 + 70.4 * 18.3;");
+
+start(m);
+
+get_register_contents(m, "val");
+
+// 1288.9286956521742

src/virtual-machines/__tests__/register-machine-explicit-control-evaluator.arithmetic-2.js

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+set_program_to_run("(1 + 3) * 4;");
+
+start(m);
+
+get_register_contents(m, "val");
+
+// 16

src/virtual-machines/__tests__/register-machine-explicit-control-evaluator.arithmetic-3.js

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+set_program_to_run("1 + 3 * 4;");
+
+start(m);
+
+get_register_contents(m, "val");
+
+// 13

src/virtual-machines/__tests__/register-machine-explicit-control-evaluator.boolean-expression.js

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+set_program_to_run("! (1 === 1);");
+
+start(m);
+
+get_register_contents(m, "val");
+
+// false

src/virtual-machines/__tests__/register-machine-explicit-control-evaluator.constant-assignment.js

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+set_program_to_run("const foo = 1; foo;");
+
+start(m);
+
+get_register_contents(m, "val");
+
+// 1

src/virtual-machines/__tests__/register-machine-explicit-control-evaluator.constant-boolean.js

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+set_program_to_run("true;");
+
+start(m);
+
+get_register_contents(m, "val");
+
+// true

src/virtual-machines/__tests__/register-machine-explicit-control-evaluator.constant-number.js

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+set_program_to_run("1;");
+
+start(m);
+
+get_register_contents(m, "val");
+
+// 1

src/virtual-machines/__tests__/register-machine-explicit-control-evaluator.function-add-two.js

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+set_program_to_run("function add(a, b) {return a + b;} add(1, 3);");
+
+start(m);
+
+get_register_contents(m, "val");
+
+// 4

src/virtual-machines/__tests__/register-machine-explicit-control-evaluator.function-factorial.js

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+set_program_to_run(
+  "function factorial(n) { return n === 1 ? 1 : n * factorial(n - 1);} factorial(4);"
+);
+
+start(m);
+
+get_register_contents(m, "val");
+
+// 24

src/virtual-machines/__tests__/register-machine-explicit-control-evaluator.function-simple-return-call.js

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+set_program_to_run("function foo() {return 1;} foo();");
+
+start(m);
+
+get_register_contents(m, "val");
+
+// 1

src/virtual-machines/__tests__/register-machine-explicit-control-evaluator.function-simple-return.js

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+set_program_to_run("function foo() {return 1;}");
+
+start(m);
+
+get_register_contents(m, "val");
+
+// 'ok'

src/virtual-machines/__tests__/register-machine-explicit-control-evaluator.long-sequence.js

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+set_program_to_run("function foo() {return 1;} 2 + 2; foo(); 2 - 3;");
+
+start(m);
+
+get_register_contents(m, "val");
+
+// -1

src/virtual-machines/__tests__/register-machine-explicit-control-evaluator.sequence.js

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+set_program_to_run("-12 - 8; 2;");
+
+start(m);
+
+get_register_contents(m, "val");
+
+// 2

src/virtual-machines/__tests__/register-machine-explicit-control-evaluator.string-concat.js

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+set_program_to_run("'hello' + ' ' + 'world';");
+
+start(m);
+
+get_register_contents(m, "val");
+
+// 'hello world'

src/virtual-machines/__tests__/register-machine-explicit-control-evaluator.ternary.js

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+set_program_to_run("(! (1 === 1)) ? 1 : 2;");
+
+start(m);
+
+get_register_contents(m, "val");
+
+// 2