From a439f15c5f240c85d4dc5a29a08dbdf88ac79d4c Mon Sep 17 00:00:00 2001
From: Yura Dupyn <2153100+omedusyo@users.noreply.github.com>
Date: Mon, 6 Apr 2026 17:17:30 +0200
Subject: [PATCH] Introduce SourceRegion

---
 src/index.ts | 50 +++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 41 insertions(+), 9 deletions(-)

diff --git a/src/index.ts b/src/index.ts
index 21fc37e..ded5edc 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -97,6 +97,22 @@ export class SourceText {
     return { index, line, column };
   }
 
+  // Creates a SourceRegion from a Span.
+  makeRegion(span: Span): SourceRegion {
+    // Basic validation
+    if (span.start.index < 0 || span.end.index > this.length) {
+      throw new Error(`Span out of bounds: ${span.start.index}-${span.end.index} (length: ${this.length})`);
+    }
+    return new SourceRegion(this, span);
+  }
+
+  // Creates a SourceRegion covering the entire SourceText.
+  fullRegion(): SourceRegion {
+    const start = this.getLocation(0);
+    const end = this.getLocation(this.length);
+    return this.makeRegion({ start, end });
+  }
+
   // Returns the full text of a specific line (1-based index)
   getLineText(line: number): string {
     const lineIndex = line - 1;
@@ -132,18 +148,29 @@ export function sourceText(s: string): SourceText {
 }
 
 export class SourceRegion {
-  // TODO
-  readonly sourceText: SourceText;
-  readonly span: Span;
+  constructor(
+    public readonly source: SourceText,
+    public readonly span: Span
+  ) {}
 
-  constructor(sourceText: SourceText, span: Span) {
-    this.sourceText = sourceText;
-    // TODO: What about span validation?
-    this.span = span;
+  get length(): number {
+    return this.span.end.index - this.span.start.index;
+  }
+
+  toString(): string {
+    return this.source.sliceByCp(this.span.start.index, this.span.end.index);
+  }
+
+  // Creates a sub-region within this region.
+  // Validates that the new span is contained within the current region.
+  subRegion(span: Span): SourceRegion {
+    if (span.start.index < this.span.start.index || span.end.index > this.span.end.index) {
+      throw new Error(`Sub-region span ${span.start.index}-${span.end.index} is not within parent region ${this.span.start.index}-${this.span.end.index}`);
+    }
+    return this.source.makeRegion(span);
   }
 }
 
-
 export type Span = {
   start: SourceLocation;
   end: SourceLocation;
@@ -190,7 +217,12 @@ export type LineView = {
   underline: string; // The literal "^^^" string for CLI usage
 };
 
-export function renderSpan(text: SourceText, span: Span, contextLines = 1): LineView[] {
+export function renderRegion(region: SourceRegion, contextLines = 1): LineView[] {
+  return renderSpan(region, region.span, contextLines);
+}
+
+export function renderSpan(region: SourceRegion, span: Span, contextLines = 1): LineView[] {
+  const text = region.source;
   const views: LineView[] = [];
   
   // Determine range of lines to show (including context)