Add slice methods, some helpers for CodePointSlice and a few predicates

src/index.ts

@@ -37,8 +37,13 @@ export function isDigit(x: CodePoint): boolean {
   return isBetween(DIGIT_0, x, DIGIT_9);
 }
 
-export function isAlphanumeric(x: CodePoint): boolean {
-  return isDigit(x) || isBetween(LOWERCASE_a, x, LOWERCASE_z) || isBetween(LOWERCASE_z, x, UPPERCASE_Z);
+export function isAsciiAlpha(x: CodePoint): boolean {
+  return isBetween(LOWERCASE_a, x, LOWERCASE_z)
+    || isBetween(UPPERCASE_A, x, UPPERCASE_Z);
+}
+
+export function isAsciiAlphanumeric(x: CodePoint): boolean {
+  return isAsciiAlpha(x) || isDigit(x);
 }
 
 export type CodePointRef = {
@@ -137,6 +142,10 @@ export class SourceText {
     return this.source.slice(startOff, endOff);
   }
 
+  slice(span: CodePointSpan): string {
+    return this.sliceByCp(span.start, span.end);
+  }
+
   // Returns a Span for the given line (1-based index).
   // If stripNewlines is true, the span will exclude trailing \r\n.
   getLineSpan(line: number, stripNewlines = true): Span {
@@ -248,6 +257,18 @@ export function rawSpan(start: CodePointIndex, end: CodePointIndex): CodePointSp
   return { start, end };
 }
 
+export function pointSpan(index: CodePointIndex): CodePointSpan {
+  return { start: index, end: index };
+}
+
+export function spanLength(span: CodePointSpan): number {
+  return span.end - span.start;
+}
+
+export function isZeroWidth(span: CodePointSpan): boolean {
+  return span.start === span.end;
+}
+
 export class SourceRegion {
   constructor(
     public readonly source: SourceText,
@@ -317,6 +338,13 @@ export class SourceRegion {
     }
   }
 
+  slice(span: CodePointSpan): string {
+    if (span.start < this.span.start.index || span.end > this.span.end.index) {
+      throw new Error(`CodePointSpan ${span.start}-${span.end} is outside region ${this.span.start.index}-${this.span.end.index}`);
+    }
+    return this.source.sliceByCp(span.start, span.end);
+  }
+
   // Creates a sub-region within this region.
   // Validates that the new span is contained within the current region.
   subRegion(span: Span): SourceRegion {