Override grapheme cluster width

Add functionality to override the grapheme cluster width. This is
useful when dealing with terminals that do not confirm to the Unicode
specification and display a grapheme cluster with the incorrect width.

By having uniseg report back the same response as the terminal, this
allows for the terminal to match cursor positioning with the
application. While the results are incorrect, this still provides a
better user experience.

Signed-off-by: Michael Lorant <[email protected]>

grapheme.go

@@ -227,7 +227,7 @@ func FirstGraphemeCluster(b []byte, state int) (cluster, rest []byte, width, new
 		} else {
 			prop = state >> shiftGraphemePropState
 		}
-		return b, nil, runeWidth(r, prop), grAny | (prop << shiftGraphemePropState)
+		return b, nil, overrideWidth(string(b), runeWidth(r, prop)), grAny | (prop << shiftGraphemePropState)
 	}
 
 	// If we don't know the state, determine it now.
@@ -250,7 +250,7 @@ func FirstGraphemeCluster(b []byte, state int) (cluster, rest []byte, width, new
 		state, prop, boundary = transitionGraphemeState(state&maskGraphemeState, r)
 
 		if boundary {
-			return b[:length], b[length:], width, state | (prop << shiftGraphemePropState)
+			return b[:length], b[length:], overrideWidth(string(b[:length]), width), state | (prop << shiftGraphemePropState)
 		}
 
 		if firstProp == prExtendedPictographic {
@@ -265,7 +265,7 @@ func FirstGraphemeCluster(b []byte, state int) (cluster, rest []byte, width, new
 
 		length += l
 		if len(b) <= length {
-			return b, nil, width, grAny | (prop << shiftGraphemePropState)
+			return b, nil, overrideWidth(string(b), width), grAny | (prop << shiftGraphemePropState)
 		}
 	}
 }
@@ -287,7 +287,7 @@ func FirstGraphemeClusterInString(str string, state int) (cluster, rest string,
 		} else {
 			prop = state >> shiftGraphemePropState
 		}
-		return str, "", runeWidth(r, prop), grAny | (prop << shiftGraphemePropState)
+		return str, "", overrideWidth(str, runeWidth(r, prop)), grAny | (prop << shiftGraphemePropState)
 	}
 
 	// If we don't know the state, determine it now.
@@ -310,7 +310,7 @@ func FirstGraphemeClusterInString(str string, state int) (cluster, rest string,
 		state, prop, boundary = transitionGraphemeState(state&maskGraphemeState, r)
 
 		if boundary {
-			return str[:length], str[length:], width, state | (prop << shiftGraphemePropState)
+			return str[:length], str[length:], overrideWidth(str[:length], width), state | (prop << shiftGraphemePropState)
 		}
 
 		if firstProp == prExtendedPictographic {
@@ -325,7 +325,7 @@ func FirstGraphemeClusterInString(str string, state int) (cluster, rest string,
 
 		length += l
 		if len(str) <= length {
-			return str, "", width, grAny | (prop << shiftGraphemePropState)
+			return str, "", overrideWidth(str, width), grAny | (prop << shiftGraphemePropState)
 		}
 	}
 }

graphemeoverrides.go

@@ -0,0 +1,15 @@
+package uniseg
+
+var GraphemeClusterWidthOverrides map[string]int
+
+func overrideWidth(gc string, w int) int {
+	if len(GraphemeClusterWidthOverrides) == 0 {
+		return w
+	}
+
+	if width, ok := GraphemeClusterWidthOverrides[gc]; ok {
+		return width
+	}
+
+	return w
+}