118 lines
4.6 KiB
Markdown
118 lines
4.6 KiB
Markdown
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---
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id: 5a23c84252665b21eecc8038
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title: Subleq
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challengeType: 5
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forumTopicId: 302328
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---
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## Description
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<section id='description'>
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<a href="https://rosettacode.org/wiki/eso:Subleq" target="_blank">Subleq</a> is an example of a <a href="https://en.wikipedia.org/wiki/One_instruction_set_computer" target="_blank">One-Instruction Set Computer (OISC)</a>.
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It is named after its only instruction, which is <b>SU</b>btract and <b>B</b>ranch if <b>L</b>ess than or <b>EQ</b>ual
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to zero.
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Your task is to create an interpreter which emulates such a machine.
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The machine's memory consists of an array of signed integers. Any reasonable word size is fine, but the memory must be
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able to hold negative as well as positive numbers.
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Execution begins with the instruction pointer aimed at the first word, which is address 0. It proceeds as follows:
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<ol>
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<li>Let A, B, and C be the value stored in the three consecutive words in memory starting at the instruction pointer.</li>
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<li>Advance the instruction pointer 3 words to point at the address after the one containing C.</li>
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<li>If A is -1, then a character is read from standard input and its code point stored in the address given by B. C
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is unused.</li>
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<li>If B is -1, then the number contained in the address given by A is interpreted as a code point and the
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corresponding character output. C is again unused.</li>
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<li>Otherwise, both A and B are treated as the addresses of memory locations. The number contained in the address
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given by A is subtracted from the number at the address given by B (and the result stored back in address B). If
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the result is zero or negative, the value C becomes the new instruction pointer.</li>
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<li>If the instruction pointer becomes negative, execution halts.</li>
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</ol>
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Other negative addresses besides -1 may be treated as equivalent to -1, or generate an error, as you see fit.
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Your solution should accept a program to execute on the machine, separately from the input fed to the program itself.
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This program should be in raw subleq "machine code" - whitespace-separated decimal numbers, with no symbolic names or
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other assembly-level extensions, to be loaded into memory starting at address 0. Show the output of your solution when
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fed this "Hello, world!" program. (Note that the example assumes ASCII or a superset of it, such as any of the Latin-N
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character sets or Unicode. You may translate it into another character set if your implementation is on a
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non-ASCiI-compatible environment.)
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<pre>15 17 -1 17 -1 -1 16 1 -1 16 3 -1 15 15 0 0 -1 72 101 108 108 111 44 32 119 111 114 108 100 33 10 0</pre>
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Which corresponds to something like this in a hypothetical assembler language:
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<pre>
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start:
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zero, message, -1
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message, -1, -1
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neg1, start+1, -1
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neg1, start+3, -1
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zero, zero, start
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zero: 0
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neg1: -1
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message: "Hello, world!\n\0"
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</pre>
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</section>
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## Instructions
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<section id='instructions'>
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Write a function that takes an array of integers as a parameter. This represents the memory elements. The function
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should interpret the sequence and return the output string. For this task, assume that there is no standard input.
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</section>
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## Tests
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<section id='tests'>
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```yml
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tests:
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- text: <code>Subleq</code> should be a function.
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testString: assert(typeof Subleq == 'function');
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- text: <code>Subleq([15, 17, -1, 17, -1, -1, 16, 1, -1, 16, 3, -1, 15, 15, 0, 0, -1, 72, 101, 108, 108, 111, 44, 32, 119, 111, 114, 108, 100, 33, 0])</code> should return a string.
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testString: assert(typeof Subleq([15, 17, -1, 17, -1, -1, 16, 1, -1, 16, 3, -1, 15, 15, 0, 0, -1, 72, 101, 108, 108, 111, 44, 32, 119, 111, 114, 108, 100, 33, 0]) == 'string');
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- text: <code>Subleq([15, 17, -1, 17, -1, -1, 16, 1, -1, 16, 3, -1, 15, 15, 0, 0, -1, 72, 101, 108, 108, 111, 44, 32, 119, 111, 114, 108, 100, 33, 0])</code> should return <code>"Hello, world!"</code>.
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testString: assert.equal(Subleq([15, 17, -1, 17, -1, -1, 16, 1, -1, 16, 3, -1, 15, 15, 0, 0, -1, 72, 101, 108, 108, 111, 44, 32, 119, 111, 114, 108, 100, 33, 0]), "Hello, world!");
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```
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</section>
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## Challenge Seed
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<section id='challengeSeed'>
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<div id='js-seed'>
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```js
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function Subleq(mem) {
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}
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```
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</div>
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</section>
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## Solution
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<section id='solution'>
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```js
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function Subleq(mem) {
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var out = '';
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var instructionPointer = 0;
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do {
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var a = mem[instructionPointer];
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var b = mem[instructionPointer + 1];
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if (a === -1) {
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} else if (b === -1) {
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out += String.fromCharCode(mem[a]);
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} else {
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mem[b] -= mem[a];
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if (mem[b] < 1) {
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instructionPointer = mem[instructionPointer + 2];
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continue;
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}
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}
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instructionPointer += 3;
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} while (instructionPointer >= 0);
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return out;
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}
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```
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</section>
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