improve generator performance

lindenmayer-system/lindenmeyer-system.js

@@ -1,8 +1,7 @@
 
-function startswith(a, b) {
-    for(let i=0; i<b.length; i++) {
-        // in JS, ['X'] == 'X'
-        if((a[i] != b[i]) | (typeof a[i] != typeof b[i])) {
+function rule_applicable(word, pos, consumed, rule) {
+    for(let i=0; i<rule.length; i++) {
+        if((word[pos+i] != rule[i]) | (consumed[pos+i] == true)) {
             return false;
         }
     }
@@ -15,37 +14,36 @@ function generate(start, rules, iterations) {
         return start;
     }
 
+    var word = start;
+    var consumed = new Array(word.length);
     for(let rule of rules) {
         let pos = 0;
-        while(pos < start.length) {
-            let w = start.slice(pos, pos + rule[0].length);
-            if(startswith(w, rule[0])) {
+        var newword = [];
+        var newconsumed = [];
+        while(pos < word.length) {
+            if(rule_applicable(word, pos, consumed, rule[0])) {
                 var ruleres;
                 if(typeof rule[1] == 'function') {
                     ruleres = rule[1]();
                 } else {
                     ruleres = rule[1];
                 }
-                start.splice(pos, rule[0].length, ruleres)
+                newword.push(...ruleres);
+                for(let i=0; i<ruleres.length; i++) {
+                    newconsumed.push(true);
+                }
                 pos += rule[0].length;
             } else {
+                newword.push(word[pos]);
+                newconsumed.push(false);
                 pos += 1;
             }
         }
+        word = newword;
+        consumed = newconsumed;
     }
     
-    // TODO find a better way
-    let ret = [];
-    for(let x of start) {
-        if(typeof x === 'object') {
-            // TODO why?
-            //ret.concat(x);
-            ret = ret.concat(x);
-        } else {
-            ret.push(x);
-        }
-    }
-    return generate(ret, rules, iterations - 1);
+    return generate(newword, rules, iterations - 1);
 }