Implement strpart()

Author: J-Fields

src/vimscript/expression/evaluate.ts

@@ -1241,7 +1241,13 @@ export class EvaluationContext {
         const [s] = getArgs(1);
         return int(toString(s!).length);
       }
-      // TODO: strpart()
+      case 'strpart': {
+        const [_src, _start, _len, chars] = getArgs(2, 4);
+        const src = toString(_src!);
+        const start = toInt(_start!);
+        const len = _len ? toInt(_len) : src.length - start;
+        return str(src.substring(start, start + len));
+      }
       // TODO: submatch()
       // TODO: substitute()
       case 'tan': {

test/vimscript/expression.test.ts

@@ -764,6 +764,13 @@ suite('Vimscript expressions', () => {
       exprTest('strlen([1,2,3])', { error: ErrorCode.UsingListAsAString });
     });
 
+    suite('strpart', () => {
+      exprTest('strpart("abcdefg", 3, 2)', { value: str('de') });
+      exprTest('strpart("abcdefg", -2, 4)', { value: str('ab') });
+      exprTest('strpart("abcdefg", 5, 4)', { value: str('fg') });
+      exprTest('strpart("abcdefg", 3)', { value: str('defg') });
+    });
+
     suite('split', () => {
       exprTest('split("  a\t\tb    c  ")', { value: list([str('a'), str('b'), str('c')]) });
       exprTest('split("  a\t\tb    c  ", "", 1)', {