feat(ansi): func to convert byte pos to char pos (#340)

* feat(ansi): func to convert byte pos to char pos

Signed-off-by: Carlos Alexandro Becker <caarlos0@users.noreply.github.com>

* docs: typo

* fix: rename

---------

Signed-off-by: Carlos Alexandro Becker <caarlos0@users.noreply.github.com>

ansi/go.mod

@@ -1,6 +1,6 @@
 module github.com/charmbracelet/x/ansi
 
-go 1.18
+go 1.21
 
 require (
 	github.com/lucasb-eyer/go-colorful v1.2.0

ansi/truncate.go

@@ -199,3 +199,28 @@ func TruncateLeft(s string, n int, prefix string) string {
 
 	return buf.String()
 }
+
+// ByteToGraphemeRange takes start and stop byte positions and converts them to
+// grapheme-aware char positions.
+// You can use this with [Truncate], [TruncateLeft], and [Cut].
+func ByteToGraphemeRange(str string, byteStart, byteStop int) (charStart, charStop int) {
+	bytePos, charPos := 0, 0
+	gr := uniseg.NewGraphemes(str)
+	for byteStart > bytePos {
+		if !gr.Next() {
+			break
+		}
+		bytePos += len(gr.Str())
+		charPos += max(1, gr.Width())
+	}
+	charStart = charPos
+	for byteStop > bytePos {
+		if !gr.Next() {
+			break
+		}
+		bytePos += len(gr.Str())
+		charPos += max(1, gr.Width())
+	}
+	charStop = charPos
+	return
+}

ansi/truncate_test.go

@@ -361,3 +361,49 @@ func TestCut(t *testing.T) {
 		})
 	}
 }
+
+func TestByteToGraphemeRange(t *testing.T) {
+	cases := []struct {
+		name   string
+		feed   [2]int
+		expect [2]int
+		input  string
+	}{
+		{
+			name:   "simple",
+			input:  "hello world from x/ansi",
+			feed:   [2]int{2, 9},
+			expect: [2]int{2, 9},
+		},
+		{
+			name:   "with emoji",
+			input:  " Downloads",
+			feed:   [2]int{4, 7},
+			expect: [2]int{2, 5},
+		},
+		{
+			name:   "start out of bounds",
+			input:  "some text",
+			feed:   [2]int{-1, 5},
+			expect: [2]int{0, 5},
+		},
+		{
+			name:   "end out of bounds",
+			input:  "some text",
+			feed:   [2]int{1, 50},
+			expect: [2]int{1, 9},
+		},
+	}
+
+	for _, tt := range cases {
+		t.Run(tt.name, func(t *testing.T) {
+			charStart, charStop := ByteToGraphemeRange(tt.input, tt.feed[0], tt.feed[1])
+			if expect := tt.expect[0]; expect != charStart {
+				t.Errorf("expected start to be %d, got %d", expect, charStart)
+			}
+			if expect := tt.expect[1]; expect != charStop {
+				t.Errorf("expected stop to be %d, got %d", expect, charStop)
+			}
+		})
+	}
+}