unicode: fix SpecialCase to follow its docs & respect explict no-op mappings

If SpecialCase contains an explicit CaseRange with zero deltas,
respect those and don't fall back to the default behavior.

Fixes #25636

Change-Id: Ic554c6b3dd462b1b39c75194eec469b6ff4aa55b
Reviewed-on: https://go-review.googlesource.com/117155
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

diff --git a/src/unicode/letter.go b/src/unicode/letter.go
index 4d9fc67165cf816ff9072ceb3f31e2ad4e1f7396..8be9a7b7c96a4ffc7c4d10d7051e1835099c432e 100644 (file)
--- a/src/unicode/letter.go
+++ b/src/unicode/letter.go
@@ -206,9 +206,10 @@ func IsTitle(r rune) bool {
 }
 
 // to maps the rune using the specified case mapping.
-func to(_case int, r rune, caseRange []CaseRange) rune {
+// It additionally reports whether caseRange contained a mapping for r.
+func to(_case int, r rune, caseRange []CaseRange) (mappedRune rune, foundMapping bool) {
        if _case < 0 || MaxCase <= _case {
-               return ReplacementChar // as reasonable an error as any
+               return ReplacementChar, false // as reasonable an error as any
        }
        // binary search over ranges
        lo := 0
@@ -229,9 +230,9 @@ func to(_case int, r rune, caseRange []CaseRange) rune {
                                // bit in the sequence offset.
                                // The constants UpperCase and TitleCase are even while LowerCase
                                // is odd so we take the low bit from _case.
-                               return rune(cr.Lo) + ((r-rune(cr.Lo))&^1 | rune(_case&1))
+                               return rune(cr.Lo) + ((r-rune(cr.Lo))&^1 | rune(_case&1)), true
                        }
-                       return r + delta
+                       return r + delta, true
                }
                if r < rune(cr.Lo) {
                        hi = m
@@ -239,12 +240,13 @@ func to(_case int, r rune, caseRange []CaseRange) rune {
                        lo = m + 1
                }
        }
-       return r
+       return r, false
 }
 
 // To maps the rune to the specified case: UpperCase, LowerCase, or TitleCase.
 func To(_case int, r rune) rune {
-       return to(_case, r, CaseRanges)
+       r, _ = to(_case, r, CaseRanges)
+       return r
 }
 
 // ToUpper maps the rune to upper case.
@@ -282,8 +284,8 @@ func ToTitle(r rune) rune {
 
 // ToUpper maps the rune to upper case giving priority to the special mapping.
 func (special SpecialCase) ToUpper(r rune) rune {
-       r1 := to(UpperCase, r, []CaseRange(special))
-       if r1 == r {
+       r1, hadMapping := to(UpperCase, r, []CaseRange(special))
+       if r1 == r && !hadMapping {
                r1 = ToUpper(r)
        }
        return r1
@@ -291,8 +293,8 @@ func (special SpecialCase) ToUpper(r rune) rune {
 
 // ToTitle maps the rune to title case giving priority to the special mapping.
 func (special SpecialCase) ToTitle(r rune) rune {
-       r1 := to(TitleCase, r, []CaseRange(special))
-       if r1 == r {
+       r1, hadMapping := to(TitleCase, r, []CaseRange(special))
+       if r1 == r && !hadMapping {
                r1 = ToTitle(r)
        }
        return r1
@@ -300,8 +302,8 @@ func (special SpecialCase) ToTitle(r rune) rune {
 
 // ToLower maps the rune to lower case giving priority to the special mapping.
 func (special SpecialCase) ToLower(r rune) rune {
-       r1 := to(LowerCase, r, []CaseRange(special))
-       if r1 == r {
+       r1, hadMapping := to(LowerCase, r, []CaseRange(special))
+       if r1 == r && !hadMapping {
                r1 = ToLower(r)
        }
        return r1

diff --git a/src/unicode/letter_test.go b/src/unicode/letter_test.go
index 3fe72ff13d24168b0130fba969be7e39deb3c0af..19ee535d57e15e3dee16fa73cf1e368781f330d9 100644 (file)
--- a/src/unicode/letter_test.go
+++ b/src/unicode/letter_test.go
@@ -9,6 +9,7 @@ import (
        "fmt"
        "runtime"
        "sort"
+       "strings"
        "testing"
        . "unicode"
 )
@@ -551,3 +552,14 @@ func TestLatinOffset(t *testing.T) {
                }
        }
 }
+
+func TestSpecialCaseNoMapping(t *testing.T) {
+       // Issue 25636
+       // no change for rune 'A', zero delta, under upper/lower/title case change.
+       var noChangeForCapitalA = CaseRange{'A', 'A', [MaxCase]rune{0, 0, 0}}
+       got := strings.ToLowerSpecial(SpecialCase([]CaseRange{noChangeForCapitalA}), "ABC")
+       want := "Abc"
+       if got != want {
+               t.Errorf("got %q; want %q", got, want)
+       }
+}