September Morn Update

Task/Execute-a-Markov-algorithm/JavaScript/execute-a-markov-algorithm.js

@@ -0,0 +1,160 @@
+/**
+ * Take a ruleset and return a function which takes a string to which the rules
+ * should be applied.
+ * @param {string} ruleSet
+ * @returns {function(string): string}
+ */
+const markov = ruleSet => {
+
+  /**
+   * Split a string at an index
+   * @param {string} s The string to split
+   * @param {number} i The index number where to split.
+   * @returns {Array<string>}
+   */
+  const splitAt = (s, i) => [s.slice(0, i), s.slice(i)];
+
+  /**
+   * Strip a leading number of chars from a string.
+   * @param {string} s The string to strip the chars from
+   * @param {string} strip A string who's length will determine the number of
+   *    chars to strip.
+   * @returns {string}
+   */
+  const stripLeading = (s, strip) => s.split('')
+      .filter((e, i) => i >= strip.length).join('');
+
+  /**
+   * Replace the substring in the string.
+   * @param {string} s The string to replace the substring in
+   * @param {string} find The sub-string to find
+   * @param {string} rep The replacement string
+   * @returns {string}
+   */
+  const replace = (s, find, rep) => {
+    let result = s;
+    if (s.indexOf(find) >= 0) {
+      const a = splitAt(s, s.indexOf(find));
+      result = [a[0], rep, stripLeading(a[1], find)].join('');
+    }
+    return result;
+  };
+
+  /**
+   * Convert a ruleset string into a map
+   * @param {string} ruleset
+   * @returns {Map}
+   */
+  const makeRuleMap = ruleset => ruleset.split('\n')
+      .filter(e => !e.startsWith('#'))
+      .map(e => e.split(' -> '))
+      .reduce((p,c) => p.set(c[0], c[1]), new Map());
+
+  /**
+   * Recursively apply the ruleset to the string.
+   * @param {Map} rules The rules to apply
+   * @param {string} s The string to apply the rules to
+   * @returns {string}
+   */
+  const parse = (rules, s) => {
+    const o = s;
+    for (const [k, v] of rules.entries()) {
+      if (v.startsWith('.')) {
+        s = replace(s, k, stripLeading(v, '.'));
+        break;
+      } else {
+        s = replace(s, k, v);
+        if (s !== o) { break; }
+      }
+    }
+    return o === s ? s : parse(rules, s);
+  };
+
+  const ruleMap = makeRuleMap(ruleSet);
+
+  return str => parse(ruleMap, str)
+};
+
+
+const ruleset1 = `# This rules file is extracted from Wikipedia:
+# http://en.wikipedia.org/wiki/Markov_Algorithm
+A -> apple
+B -> bag
+S -> shop
+T -> the
+the shop -> my brother
+a never used -> .terminating rule`;
+
+const ruleset2 = `# Slightly modified from the rules on Wikipedia
+A -> apple
+B -> bag
+S -> .shop
+T -> the
+the shop -> my brother
+a never used -> .terminating rule`;
+
+const ruleset3 = `# BNF Syntax testing rules
+A -> apple
+WWWW -> with
+Bgage -> ->.*
+B -> bag
+->.* -> money
+W -> WW
+S -> .shop
+T -> the
+the shop -> my brother
+a never used -> .terminating rule`;
+
+const ruleset4 = `### Unary Multiplication Engine, for testing Markov Algorithm implementations
+### By Donal Fellows.
+# Unary addition engine
+_+1 -> _1+
+1+1 -> 11+
+# Pass for converting from the splitting of multiplication into ordinary
+# addition
+1! -> !1
+,! -> !+
+_! -> _
+# Unary multiplication by duplicating left side, right side times
+1*1 -> x,@y
+1x -> xX
+X, -> 1,1
+X1 -> 1X
+_x -> _X
+,x -> ,X
+y1 -> 1y
+y_ -> _
+# Next phase of applying
+1@1 -> x,@y
+1@_ -> @_
+,@_ -> !_
+++ -> +
+# Termination cleanup for addition
+_1 -> 1
+1+_ -> 1
+_+_ -> `;
+
+const ruleset5 = `# Turing machine: three-state busy beaver
+#
+# state A, symbol 0 => write 1, move right, new state B
+A0 -> 1B
+# state A, symbol 1 => write 1, move left, new state C
+0A1 -> C01
+1A1 -> C11
+# state B, symbol 0 => write 1, move left, new state A
+0B0 -> A01
+1B0 -> A11
+# state B, symbol 1 => write 1, move right, new state B
+B1 -> 1B
+# state C, symbol 0 => write 1, move left, new state B
+0C0 -> B01
+1C0 -> B11
+# state C, symbol 1 => write 1, move left, halt
+0C1 -> H01
+1C1 -> H11`;
+
+console.log(markov(ruleset1)('I bought a B of As from T S.'));
+console.log(markov(ruleset2)('I bought a B of As from T S.'));
+console.log(markov(ruleset3)('I bought a B of As W my Bgage from T S.'));
+console.log(markov(ruleset4)('_1111*11111_'));
+console.log(markov(ruleset5)('000000A000000'));