Parsing ANSI/VT Terminal Keycode Sequences
                        ==========================================

Parsing input from an ANSI keyboard stream can be optimised by combining all keycode
sequences into a superset that represents all sources:

    <esc> {'?' or 'O' or '['} {<num>{;<num>}} {'@'-'~'}

This can be parsed with the following 'C' code. It returns a character code < 0x100
for normal character keys, 0x180+n for function keys, and 0x1C0+n for editing keys.
With function and editing keys, the modifier keys are returned in bit 4 and bit 5.

  unsigned char ansikey[]={                /* Translation table                 */
  0x00,0xC8,0xC6,0xC7,0xC9,0xCB,0xCA,0xC8, /* 0,Home,Ins,Del,End,PgUp,PgDn,Home */
  0xC9,0x00,0x80,0x81,0x82,0x83,0x84,0x85, /* End,(9),F0,F1,F2,F3,F4,F5         */
  0x00,0x86,0x87,0x88,0x89,0x8A,0x00,0x8B, /* (16),F6,F7,F8,F9,F10,(22),F11     */
  0x8C,0x8D,0x8E,0x00,0x8F,0x00,0x00,0x00, /* F12,F13,F14,(27),F15,F16,(30),F17 */
  0x80,0x00,0xC3,0x00,                     /* F18,F19,F20,(35)                  */
  0x84,0xCF,0xCE,0xCD,0xCC,0xC5,0xC9,0x00, /* f4,Up,Dwn,Rgt,Lft,Bgn,End,Nxt     */
  0xC8,0x09,0x00,0x95,0x94,0x0D,0x00,0x00, /* Hme,Tab,Clr,Shf5,Shf4,Kent,(N),(O)*/
  0x81,0x82,0x83,0x84,0x85,0xCA,0xCB,0x00, /* f1,f2,f3,f4,f5,PgDn,PgUp,(W)      */
  0x3D,0x00,0xD5,0x00,0x00,0x00,0x00,0x00  /* K=,(Y),ShTAB,([),(\),(]),(^),(_)  */
  }

  int ch=0;				/* Character pressed			*/
  int key=0;				/* Key lookup				*/
  int mod=0;				/* Modifier=<none>			*/
  
  fflush(stdout);
  read(STDIN_FILENO, &ch, 1);		/* Read without flushing		*/
  ch=ch & 0xFF;
  if (ch != 27)     return ch;		/* Not <esc>				*/
  if (kbhit() == 0) return ch;		/* Nothing pending			*/
  
  /* Read an ANSI/VT key sequence as:
   *    <esc> [ (<num>) (;<num>) <0x40-0x7F>
   * or <esc> O <0x40-0x7F>
   * or <esc> ? <0x40-0x7F>
   * or <esc> <0x40-0x7F>
   */

  ch=getchar();
  if (ch=='O' || ch='?') {		/* Convert <esc>O or <esc>? to <esc>[1	*/
    ch=getchar();			/* Get <esc>?<char> or <esc>O<char>	*/
  } else {
    if (ch == '[') {			/* Parse <esc>[(<nums...><letter>	*/
      while ((ch=getchar())<'@') {	/* Parse through non-alphas		*/
        if (ch>='0' && ch<='9') {	/* Digit, add to current num		*/
          mod=mod*10+(ch-'0');
        }
        if (ch==';') {			/* Semicolon, next number		*/
          key=mod; mod=0;
        }
      }
      if (mod) mod=mod-1;		/* Convert modifiers to bitmap		*/
    }
  }
  if (ch<'@' || ch>0x7F) return ch;	/* Not ...<letter>, return <char>	*/

  /* <esc><char>                mod=0, key=0, ch=<char>
     <esc>?<char>               mod=0, key=0, ch=<char>
     <esc>O<char>               mod=0, key=0, ch=<char>
     <esc>[<char>               mod=0, key=0, ch=<char>
     <esc>[<num><char>          mod=<num>, key=0, ch=<char>
     <esc>[<num1>;<num2><char>  mod=<num2>, key=<num1>, ch=<char>
   */

  if (key == 0) {
    if (ch >= '`') return (ch & 0x3F) | 0x100;	/* <esc>?<lower> or <esc>O<lower> */
    else		key=36+(ch & 0x1F);	/* cursors/etc -> key 36+	  */
  }
  ch=ansikey[key];			/* Translate keypress			*/
  if (mod & 1) ch=ch ^ 0x10;		/* SHIFT pressed			*/
  if (mod & 4) ch=ch ^ 0x20;		/* CTRL pressed				*/
  if (mod & 2) ch=ch ^ 0x30;		/* ALT pressed				*/
  return (ch | 0x100);

Example code:
* JGH Console Library         - mdfs.net/System/C/Lib
* PDP11 ANSI parsing          - mdfs.net/Info/Comp/PDP11/ProgTips
* PDP11 BBC BASIC Console I/O - mdfs.net/Software/PDP11/BBCBASIC

References
----------
* invisible-island.net/xterm/ctlseqs/ctlseqs.html
* www.gnu.org/software/screen/manual/html_node/Input-Translation.html#Input-Translation

Updates
-------
22-Jun-2020: Testing suggests malformed sequences are ignored. Slight formatting changes.