#include "input.h"

// Converts decimal values of a uint32_t argument to hex.
uint32_t hex_to_decimal(const uint32_t decimal) {
    uint32_t ret = 0;
    for (size_t i = 0; i < sizeof(uint32_t); ++i) { // Iterate through 4 bytes.
        uint32_t tmp = (decimal & (0xFFu << 8 * i)) >> 8 * i;
        if (isalpha(tmp)) {
            tmp -= 'a';
            tmp += 10;
        } else if (isdigit(tmp)) {
            tmp -= '0';
        }
        // There should be no other cases if the input is hex.
        ret |= (tmp << 4 * i);
    }
    return ret;
}

// Returns the next instruction in hex. Returns 0 if EOF reached.
uint32_t get_next_instruction(FILE *const iptr) {
    uint32_t ret = 0;
    int i = 0;
    for (int c = fgetc(iptr); c != '\n'; c = fgetc(iptr), ++i) {
        if (c == EOF) {
            return 0u;
        }
        uint32_t tmp = hex_to_decimal((uint32_t)c);
        ret |= (tmp << (28 - i * 4));
    }
    // We have to take into account the potential for the hex value to not be
    // "full", ie we may read abc instead of 00000abc.
    ret >>= (32 - i * 4);
    return ret;
}

// Sets the iptr to the previous instruction, which sits on the newline so that
// later calls to get_next_instruction work as expected.
// Also, if unable to find a previous instruction, sets the iptr to EOF.
void goto_previous_instruction(FILE *const iptr) {
    int c = 0;
    fseek(iptr, -2L, SEEK_CUR);
    while (ftell(iptr)) {
        c = fgetc(iptr);
        if (c == '\n') {
            return;
        }
        fseek(iptr, -2L, SEEK_CUR);
    }
    // Start of file reached, out of bounds, so set EOF.
    fseek(iptr, 0L, SEEK_END);
}