Replace string in file (similar to MariaDB/MySQL replace utility)

replace.c

// This is free and unencumbered software released into the public domain.

// Anyone is free to copy, modify, publish, use, compile, sell, or
// distribute this software, either in source code form or as a compiled
// binary, for any purpose, commercial or non-commercial, and by any
// means.

// In jurisdictions that recognize copyright laws, the author or authors
// of this software dedicate any and all copyright interest in the
// software to the public domain. We make this dedication for the benefit
// of the public at large and to the detriment of our heirs and
// successors. We intend this dedication to be an overt act of
// relinquishment in perpetuity of all present and future rights to this
// software under copyright law.

// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
// IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
// OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
// ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
// OTHER DEALINGS IN THE SOFTWARE.

// For more information, please refer to <http://unlicense.org/>

// Usage: ./replace "foobar" "foobaz" < input_file > output_file
// Note: input_file and output_file should be different

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

typedef struct string_t {
    const char * value;
    size_t length;
} string;

struct parser_t {
    string match_text, replace_text;

    char * match_buffer;
    unsigned int match_buffer_index;

    enum { STATE_INVALID, STATE_IN, STATE_OUT } state;
};

void parser_init(struct parser_t * parser, 
                 const char * match_text,
                 const char * replace_text)
{
    memset(parser, 0, sizeof(struct parser_t));

    parser->match_text.value = match_text;
    parser->match_text.length = strlen(match_text);
    parser->replace_text.value = replace_text;
    parser->replace_text.length = strlen(replace_text);
    parser->state = STATE_OUT;
    parser->match_buffer = malloc(parser->match_text.length);
}

void parser_free(struct parser_t * parser)
{
    free(parser->match_buffer);
}

void output_char(char current_char)
{
    fwrite(&current_char, sizeof(char), 1, stdout);
}

void buffer_match(struct parser_t * parser, char current_char)
{
    parser->match_buffer[parser->match_buffer_index++] = current_char;
}

void buffer_clear(struct parser_t * parser)
{
    parser->match_buffer_index = 0;
}

void buffer_flush(struct parser_t * parser)
{
    if (parser->match_buffer_index > 0) {
        fwrite(parser->match_buffer, sizeof(char), parser->match_buffer_index, stdout);
        buffer_clear(parser);
    }
}

int process_state_in(struct parser_t * parser, char current_char)
{
    if (parser->match_text.value[parser->match_buffer_index] == current_char) {
        buffer_match(parser, current_char);

        return STATE_IN;
    }

    if (parser->match_buffer_index == parser->match_text.length) {
        fwrite(parser->replace_text.value, sizeof(char), parser->replace_text.length, stdout);
        buffer_clear(parser);

        output_char(current_char);

        return STATE_OUT;
    }

    if (parser->match_text.value[parser->match_buffer_index] != current_char) {
        buffer_flush(parser);
        output_char(current_char);

        return STATE_OUT;
    }

    return STATE_INVALID;
}

int process_state_out(struct parser_t * parser, char current_char)
{
    if (parser->match_text.value[parser->match_buffer_index] == current_char) {
        buffer_match(parser, current_char);

        return STATE_IN;
    } 

    if (parser->match_text.value[parser->match_buffer_index] != current_char) {
        buffer_flush(parser);
        output_char(current_char);

        return STATE_OUT;
    }

    return STATE_INVALID;
}

int main(int argc, char *argv[])
{
    char current_char;
    struct parser_t parser;

    if (argc != 3) {
        fprintf(stdout, "Usage:\n\t%s match_text replace_text < in_file > out_file\n\t# note in_file and out_file should be different.\n", argv[0]);
        return 0;
    }

    parser_init(&parser, argv[1], argv[2]);

    while (fread(&current_char, sizeof(char), 1, stdin) != 0) {
        switch (parser.state) {
            case STATE_IN:
            {
                parser.state = process_state_in(&parser, current_char);
            }
            break;
            case STATE_OUT:
            {
                parser.state = process_state_out(&parser, current_char);
            }
            break;
            default:
                fprintf(stderr, "Error: Invalid state.\n");
                return -1;
            break;
        }
    }

    parser_free(&parser);

    return 0;
}