/* Draw some ANSI text with a gradient from blue to orange through white.
 *
 * Truecolor escape sequences from ISO-8613-3 are supported in konsole
 * for a few years now, but in libvte (and thus gnome-terminal,
 * mate-terminal, xfce-terminal, etc.) only since 2014 and in xterm
 * only since version 282.  Older terminals will probably interpret
 * these sequences as something different.
 *
 * Here’s a screenshot of the output in konsole:
begin 644 gradient.png
MB5!.1PT*&@H````-24A$4@```"4````."`8```"H]=7V```"$TE$051(QV-@
M8&#X/P@QB)`&8G:2-#(S,?\W4#$`8Q5I%30Y)J"X!!BK2`N1ZZ@[0&Q!DD8N
M=J[_*^I6_#\R^<C_`Q,.H,FQ`N5"@'))0+D$^CD*AD/L0S`<A9#3HM11(4`\
M`XB7`;$97`$C`^-_8[54H`7+_WM;3/TOR*M$L:/86!C^ET<R_%_=P/"_*YWA
MOP@_3D<=!6(O(&X$XL=`S`16X&4QY7^LVZ[_"A+V8,<5A3WYS\[*1Y&C%E<Q
M_-_:P?#?7I_A?T\FP_\K\QG^,S%B=5085(`1B+^#$S\WA]C_VKA?P#0B#-<0
MYK@&F(`3R'84+Q?#_[][&8">A,8$T#&/5S'\=S3$ZBA+),$W0*SQ7T[<YG]=
MW.__60%7_F<'7/V?'7CM?VG$J_]V^C5D.TI+GN'_K]VHZO;W,_Q/\B28T"&.
M$A70`CL"7Z(,M`W\?W#B01QRFD"Y1!0Q&5&&___W,_SG8$.(G9G)\#_8CDA'
M,3&Q_,\)O/Y?5RD**?BE_O-PBL/Y^LKZ_U^L>P$4Y\5PE+ZR!%"N!"B'*`-!
M:>?N,H;_B="0T02&W+<=$,<2Y2@06T)('QQ]()SI?^E_4>ACH)@!BN43<B;\
M?[/Q#1CS<?&AR7D`Q<O`F`_J.`<#AO_/UC#\/S>+X?_;30S_L_RQYC["F(=3
MXC\_MRRXB*!&50(*,7EQ!F!.QEE.#<JZ;W!A`&=SF&"AC'A<`````$E%3D2N
#0F""
`
end
 */

#include <stdint.h>
#include <stdio.h>
#include <string.h>

void usage(const char *argv0) {
  fprintf(stderr, "Usage: %s message\n", argv0);
  fprintf(stderr,
          "Displays `message` in a color gradient, blue to white to orange.\n");
}

int red(float f) {
  return (f > 0.5 ? 255 : (int)(0.5 + f * 2 * 255));
}

int green(float f) {
  return (int)(0.5 + red(f) * 0.75);
}

int blue(float f) {
  return red(1 - f);
}

/* This is enough to cover the BMP so who cares */
uint8_t utf_8_len[] = {
  1, 1, 1, 1,
  1, 1, 1, 1, /* ...0111 */
  1, 1, 1, 1, /* 1000, 1001, 1010, 1011 */
  2, 2, 3, 4, /* 1100, 1101, 1110, 1111 */
};

typedef struct {
  char *p, *e;
  uint8_t bytes_left;           /* in this character */
} u8iter;

int u8iter_is_empty(u8iter *p) {
  return p->p == p->e;
}

void u8iter_embrace_nul_terminated(u8iter *p, char *s) {
  p->p = s;
  p->e = s + strlen(s);
  p->bytes_left = 0;
}

int u8iter_next_byte(u8iter *p, char *c) {
  if (!p->bytes_left) return 0;
  p->bytes_left--;
  *c = *p->p;
  p->p++;
  return 1;
}

void u8iter_next_codepoint(u8iter *p) {
  if (u8iter_is_empty(p)) return;
  while (p->bytes_left) {
    char c;
    u8iter_next_byte(p, &c);
  }
  if (!u8iter_is_empty(p)) {
    p->bytes_left = utf_8_len[(int)((uint8_t)*p->p >> 4)];
  }
}

int utf_8_strlen(char *utf8z) {
  u8iter it;
  int n = 0;
  u8iter_embrace_nul_terminated(&it, utf8z);
  while (!u8iter_is_empty(&it)) {
    u8iter_next_codepoint(&it);
    n++;
  }
  return n;
}

#define CSI "\033["
#define RGB CSI "38;2;"

void rgb(int r, int g, int b, char *dest) {
  /* This kind of use of sprintf is often insecure.  Do not imitate. */
  sprintf(dest, RGB "%d;%d;%dm", r, g, b);
}

int main(int argc, char **argv) {
  if (argc != 2) {
    usage(argv[0]);
    return 1;
  }
  int n = utf_8_strlen(argv[1]);
  u8iter it;
  u8iter_embrace_nul_terminated(&it, argv[1]);

  for (int i = 0; i != n; i++) {
    float f = (float)i/(n-1);
    char rgbbuf[32];
    rgb(red(f), green(f), blue(f), rgbbuf);
    fputs(rgbbuf, stdout);
    u8iter_next_codepoint(&it);
    char c;
    while (u8iter_next_byte(&it, &c)) {
      putchar(c);
    }
  }
  fputs(CSI "0m\n", stdout);
  return 0;
}