introduce encoding_is_utf8() internal

src/lib/notcurses.c

@@ -1095,6 +1095,30 @@ int ncplane_destroy_family(ncplane *ncp){
  return ret;
 }
 
+// glibc's _nl_normalize_charset() converts to lowercase, removing everything
+// but alnums. furthermore, "cs" is a valid prefix meaning "character set".
+static bool
+encoding_is_utf8(const char *enc){
+ if(tolower(enc[0]) == 'c' && tolower(enc[1]) == 's'){ // strncasecmp() isn't ansi/iso
+ enc += 2; // skip initial "cs" if present.
+ }
+ const char utfstr[] = "utf8";
+ const char* match = utfstr;
+ while(*enc){
+ if(isalnum(*enc)){ // we only care about alnums
+ if(tolower(*enc) != tolower(*match)){
+ return false;
+ }
+ ++match;
+ }
+ ++enc;
+ }
+ if(*match){
+ return false;
+ }
+ return true;
+}
+
 // it's critical that we're using UTF-8 encoding if at all possible. since the
 // client might not have called setlocale(2) (if they weren't reading the
 // directions...), go ahead and try calling setlocale(LC_ALL, "") and then
@@ -1112,7 +1136,7 @@ void init_lang(void){
  }
 #endif
  const char* encoding = nl_langinfo(CODESET);
- if(encoding && !strcmp(encoding, "UTF-8")){
+ if(encoding && encoding_is_utf8(encoding)){
  return; // already utf-8, great!
  }
  const char* lang = getenv("LANG");
@@ -1127,13 +1151,13 @@ void init_lang(void){
  }
 #endif
  encoding = nl_langinfo(CODESET);
- if(encoding && !strcmp(encoding, "UTF-8")){
+ if(encoding && encoding_is_utf8(encoding)){
  loginfo("set locale from LANG; client should call setlocale(2)!");
  return;
  }
  setlocale(LC_CTYPE, "C.UTF-8");
  encoding = nl_langinfo(CODESET);
- if(encoding && !strcmp(encoding, "UTF-8")){
+ if(encoding && encoding_is_utf8(encoding)){
  loginfo("forced UTF-8 encoding; client should call setlocale(2)!");
  return;
  }
@@ -1214,7 +1238,7 @@ notcurses_early_init(const struct notcurses_options* opts, FILE* fp, unsigned* u
  }
 //fprintf(stderr, "getenv LC_ALL: %s LC_CTYPE: %s\n", getenv("LC_ALL"), getenv("LC_CTYPE"));
  const char* encoding = nl_langinfo(CODESET);
- if(encoding && !strcmp(encoding, "UTF-8")){
+ if(encoding && encoding_is_utf8(encoding)){
  *utf8 = true;
  }else{
  *utf8 = false;