Question

How to compress and decompress repeated string characters by storing the repeated length (RLE) in PHP

Answer 1

/* ---------------------------------------------------------
   Compress: turn "aaabbcccc" into "a3b2c4"
   This is run-length encoding (RLE)
   --------------------------------------------------------- */
function compress_string(string $s): string {
    if ($s === "") {
        return "";   // Edge case: empty string
    }

    $out = "";        // Result string
    $count = 1;       // Count of repeated characters
    $len = strlen($s);

    // Start from index 1 and compare with previous character
    for ($i = 1; $i <= $len; $i++) {

        // If still repeating the same character, increase count
        if ($i < $len && $s[$i] === $s[$i - 1]) {
            $count++;
        } else {
            // Character changed OR reached end of string
            $out .= $s[$i - 1];   // Add the character
            $out .= $count;       // Add how many times it repeated
            $count = 1;           // Reset counter
        }
    }

    return $out;
}

/* ---------------------------------------------------------
   Decompress: turn "a3b2c4" into "aaabbcccc"
   Reads a letter, then reads digits, expands them.
   --------------------------------------------------------- */
function decompress_string(string $s): string {
    $out = "";                 // Result string
    $currentChar = null;       // The character being processed
    $number = "";              // Digits representing the count

    $len = strlen($s);

    for ($i = 0; $i < $len; $i++) {
        $c = $s[$i];

        if (ctype_alpha($c)) {   // Found a letter
            // If we already have a previous letter + number, expand it
            if ($currentChar !== null && $number !== "") {
                $count = intval($number);
                $out .= str_repeat($currentChar, $count);
            }

            $currentChar = $c;   // Start new character
            $number = "";        // Reset number buffer
        }
        else if (ctype_digit($c)) {  // Found a digit
            $number .= $c;           // Build multi-digit number
        }
    }

    // Handle the last character + number pair
    if ($currentChar !== null && $number !== "") {
        $count = intval($number);
        $out .= str_repeat($currentChar, $count);
    }

    return $out;
}

function main() {
    $s = "wwwwwwwwwwwwbwwwwwwwwwwbbbwwwwwwccccwwwwwwwwwwww";

    $c = compress_string($s);
    $d = decompress_string($c);

    echo "Original:   $s\n";
    echo "Compressed: $c\n";
    echo "Decompressed: $d\n";
}

main();


/*
run:

Original:   wwwwwwwwwwwwbwwwwwwwwwwbbbwwwwwwccccwwwwwwwwwwww
Compressed: w12b1w10b3w6c4w12
Decompressed: wwwwwwwwwwwwbwwwwwwwwwwbbbwwwwwwccccwwwwwwwwwwww

*/

 

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