Windows: encode and decode text as UTF-8 instead of trusting the locale

setlocale(LC_ALL, "") maps to the ANSI code page on Windows -- 936 (GBK)
on a Chinese system -- so wcrtomb() produces GBK where the protocol wants
UTF-8, and mbrtowc() reads UTF-8 as GBK. Typed Chinese therefore reached
the model as mojibake, and once the buffer held correct UTF-8 the input
line rendered as mojibake instead. Windows' C runtime has no ".UTF-8"
locale to switch to, so the conversion has to be explicit.

os_compat.[ch]
  * add os_utf8_encode() and os_utf8_decode(), locale-independent, with
    range and overlong-form checks
src/tui.c
  * input_put_wc(), the command-palette filter and the ask_user buffer use
    os_utf8_encode() instead of wcrtomb()
src/textsel.c
  * cst_utf8_decode() uses os_utf8_decode() instead of mbrtowc()

src/os_compat.c

@@ -1169,6 +1169,74 @@ void os_proc_kill(os_proc_t *p) {
 }
 #endif
 
+/* --- UTF-8 ---------------------------------------------------------------
+ * See the declaration in os_compat.h for why wcrtomb() is not used. */
+int os_utf8_encode(unsigned long cp, char out[4]) {
+  if (cp < 0x80) {
+    out[0] = (char)cp;
+    return 1;
+  }
+  if (cp < 0x800) {
+    out[0] = (char)(0xC0 | (cp >> 6));
+    out[1] = (char)(0x80 | (cp & 0x3F));
+    return 2;
+  }
+  if (cp >= 0xD800 && cp <= 0xDFFF)
+    return 0; /* lone surrogate half from a 16-bit wchar_t */
+  if (cp < 0x10000) {
+    out[0] = (char)(0xE0 | (cp >> 12));
+    out[1] = (char)(0x80 | ((cp >> 6) & 0x3F));
+    out[2] = (char)(0x80 | (cp & 0x3F));
+    return 3;
+  }
+  if (cp <= 0x10FFFF) {
+    out[0] = (char)(0xF0 | (cp >> 18));
+    out[1] = (char)(0x80 | ((cp >> 12) & 0x3F));
+    out[2] = (char)(0x80 | ((cp >> 6) & 0x3F));
+    out[3] = (char)(0x80 | (cp & 0x3F));
+    return 4;
+  }
+  return 0;
+}
+
+/* See the declaration in os_compat.h for why mbrtowc() is not used. */
+int os_utf8_decode(const char *s, size_t avail, unsigned long *cp) {
+  const unsigned char *p = (const unsigned char *)s;
+  int need, i;
+  unsigned long v;
+  static const unsigned long MINV[4] = {0, 0x80, 0x800, 0x10000};
+
+  if (!s || !cp || avail == 0)
+    return 0;
+  if (p[0] < 0x80) {
+    *cp = p[0];
+    return 1;
+  }
+  if ((p[0] & 0xE0) == 0xC0) {
+    need = 1;
+    v = p[0] & 0x1Fu;
+  } else if ((p[0] & 0xF0) == 0xE0) {
+    need = 2;
+    v = p[0] & 0x0Fu;
+  } else if ((p[0] & 0xF8) == 0xF0) {
+    need = 3;
+    v = p[0] & 0x07u;
+  } else {
+    return 0; /* not a UTF-8 lead byte */
+  }
+  if (avail < (size_t)need + 1)
+    return 0; /* truncated */
+  for (i = 1; i <= need; i++) {
+    if ((p[i] & 0xC0) != 0x80)
+      return 0; /* bad continuation byte */
+    v = (v << 6) | (p[i] & 0x3Fu);
+  }
+  if (v < MINV[need] || (v >= 0xD800 && v <= 0xDFFF) || v > 0x10FFFF)
+    return 0; /* overlong, surrogate, or out of range */
+  *cp = v;
+  return need + 1;
+}
+
 /* --- libc gaps on Windows -----------------------------------------------
  * strcasestr() is a GNU extension and setenv()/unsetenv() are POSIX; glibc
  * has all three, MinGW-w64 has none of them. src/tui.c needs them (command

src/textsel.c

@@ -3,23 +3,29 @@
 #endif
 
 #include "cst_textsel.h"
+#include "os_compat.h"
 
 #include <stdlib.h>
 #include <string.h>
 #include <wctype.h>
 
 int cst_utf8_decode(const char *s, size_t avail, wchar_t *wc, size_t *eaten) {
+  unsigned long cp = 0;
+  int n;
   if (avail == 0)
     return -1;
-  mbstate_t st;
-  memset(&st, 0, sizeof(st));
-  size_t n = mbrtowc(wc, s, avail, &st);
-  if (n == (size_t)-1 || n == (size_t)-2 || n == 0) {
-    *wc = (unsigned char)s[0];
+  /* os_utf8_decode(), not mbrtowc(): the latter follows setlocale(), which on
+   * Windows means the ANSI code page, so UTF-8 text would be decoded as GBK
+   * and rendered as mojibake. */
+  n = os_utf8_decode(s, avail, &cp);
+  if (n <= 0) {
+    /* Not a valid sequence: fall back to one raw byte, as before. */
+    *wc = (wchar_t)(unsigned char)s[0];
     *eaten = 1;
     return 0;
   }
-  *eaten = n;
+  *wc = (wchar_t)cp;
+  *eaten = (size_t)n;
   return 0;
 }
 

src/tui.c

@@ -2966,12 +2966,10 @@ static void input_clear(tui_t *t) { input_set(t, ""); }
 
 static void input_put_wc(tui_t *t, wchar_t wc) {
   char mb[8];
-  mbstate_t st;
-  memset(&st, 0, sizeof(st));
-  size_t n = wcrtomb(mb, wc, &st);
-  if (n == (size_t)-1 || n == 0)
+  int n = os_utf8_encode((unsigned long)wc, mb);
+  if (n <= 0)
     return;
-  input_insert(t, mb, n);
+  input_insert(t, mb, (size_t)n);
 }
 
 static void input_put_byte(tui_t *t, char ch) { input_insert(t, &ch, 1); }
@@ -3600,16 +3598,14 @@ static int picker_run(tui_t *t, const char *title, char items[][128], int n,
       continue;
     }
     if (iswprint(wch)) {
-      char mb[8];
-      mbstate_t st;
-      memset(&st, 0, sizeof(st));
-      size_t k = wcrtomb(mb, (wchar_t)wch, &st);
-      if (k != (size_t)-1 && qlen + k < sizeof(query)) {
-        memcpy(query + qlen, mb, k);
-        qlen += k;
-        query[qlen] = 0;
-        sel = top = 0;
-      }
+          char mb[8];
+          int k = os_utf8_encode((unsigned long)(wchar_t)wch, mb);
+          if (k > 0 && qlen + (size_t)k < sizeof(query)) {
+            memcpy(query + qlen, mb, (size_t)k);
+            qlen += (size_t)k;
+            query[qlen] = 0;
+            sel = top = 0;
+          }
     }
   }
 
@@ -6143,15 +6139,13 @@ static void run_ask_modal(tui_t *t) {
       break;
     }
     if (iswprint(wch)) {
-      char mb[8];
-      mbstate_t st;
-      memset(&st, 0, sizeof(st));
-      size_t k = wcrtomb(mb, (wchar_t)wch, &st);
-      size_t n = strlen(t->ask_buf);
-      if (k != (size_t)-1 && n + k + 1 < sizeof(t->ask_buf)) {
-        memcpy(t->ask_buf + n, mb, k);
-        t->ask_buf[n + k] = 0;
-      }
+          char mb[8];
+          int k = os_utf8_encode((unsigned long)(wchar_t)wch, mb);
+          size_t n = strlen(t->ask_buf);
+          if (k > 0 && n + (size_t)k + 1 < sizeof(t->ask_buf)) {
+            memcpy(t->ask_buf + n, mb, (size_t)k);
+            t->ask_buf[n + k] = 0;
+          }
     }
   }
   curs_set(1);