Morse code encoder

morse2.c

// from morse.tc - 9/29/2012, 7/20/19, 8/21,22/21 - tag, sbg, lrb
#include <string.h>
#include <stdio.h>

// https://kb8ojh.net/msp430/morse_encoding.html
#define NUL 0
//#define FULLTABLE
const unsigned char morse_ascii[] = {
#ifdef FULLTABLE
     NUL,  NUL,  NUL,  NUL,
     NUL,  NUL,  NUL,  NUL,
     NUL,  NUL,  NUL,  NUL,
     NUL,  NUL,  NUL,  NUL,
     NUL,  NUL,  NUL,  NUL,
     NUL,  NUL,  NUL,  NUL,
     NUL,  NUL,  NUL,  NUL,
     NUL,  NUL,  NUL,  NUL,
     NUL,  NUL,  NUL,  NUL,
     NUL,  NUL,  NUL,  NUL,
     NUL,  NUL,  NUL,  NUL,
#endif
    0x73,  NUL, 0x55, 0x32,  /* , _ . / */
    0x3F, 0x2F, 0x27, 0x23,  /* 0 1 2 3 */
    0x21, 0x20, 0x30, 0x38,  /* 4 5 6 7 */
    0x3C, 0x3E,  NUL,  NUL,  /* 8 9 _ _ */
     NUL, 0x31,  NUL, 0x4C,  /* _ = _ ? */
     NUL, 0x05, 0x18, 0x1A,  /* _ A B C */
    0x0C, 0x02, 0x12, 0x0E,  /* D E F G */
    0x10, 0x04, 0x17, 0x0D,  /* H I J K */
    0x14, 0x07, 0x06, 0x0F,  /* L M N O */
    0x16, 0x1D, 0x0A, 0x08,  /* P Q R S */
    0x03, 0x09, 0x11, 0x0B,  /* T U V W */
    0x19, 0x1B, 0x1C,  NUL,  /* X Y Z _ */
#ifdef FULLTABLE
     NUL,  NUL,  NUL,  NUL,
     NUL, 0x05, 0x18, 0x1A,  /* _ A B C */
    0x0C, 0x02, 0x12, 0x0E,  /* D E F G */
    0x10, 0x04, 0x17, 0x0D,  /* H I J K */
    0x14, 0x07, 0x06, 0x0F,  /* L M N O */
    0x16, 0x1D, 0x0A, 0x08,  /* P Q R S */
    0x03, 0x09, 0x11, 0x0B,  /* T U V W */
    0x19, 0x1B, 0x1C,  NUL,  /* X Y Z _ */
     NUL,  NUL,  NUL,  NUL,
#endif
};

/*// this implementation was error-prone to write
char *morseStr1(char c)
{
    unsigned char b = morse_ascii[c];
    if (b ==  NUL) return "";
    static char buf[8] = ""; // not thread-safe
    char *p = buf;
    int i = 8; // bits in a byte
    while (i-- , (b & 0x80) != 0x80 )
        b <<= 1;
    while (i--) {
        b <<= 1;
        *p++ = (b&0x80)?'_':'.';
    }
    *p = '\0';
    return buf;
}
*/

// this seemed easier to write, and worked the first time
// moved buf to an argument, for thread-safety
char *morseStr(char c, char *buf)
{
#ifndef FULLTABLE
    c -= 44 + (c>='a'?32:0);
    if (c < 0 || c >= sizeof(morse_ascii))
        c = sizeof(morse_ascii)-1;
#endif    
    unsigned char b = morse_ascii[c];
    if (b ==  NUL) 
        return "";
    char *p = buf;
    char mask = 0x40; // no more than 6 codes for each character, so no more than 7 bits total
    // find MSB one bit:
    while (!(b & mask))
        mask >>= 1;
    // remaining bits are morse codes
    // shift first to move past the sentinel bit
    while (mask >>= 1) {
        *p++ = (b & mask) ? '_' : '.';
    }
    *p = '\0';
    return buf;
}

int main() {
    puts("enter phrase to convert to Morse code");
    char s[600];
    fgets(s, 600, stdin);
    char c;
    char buf[7]; 
    for (char *p = s; (c = *p) != 0; p++)
    {
        printf("%c %s  ", c, morseStr(c, buf) );
    }
}