re-implement the uuid v7 uuid7 uuidv7 from uuidgen (part of https://github.com/util-linux/util-linux ) as version 2.41 using source code as context into bash shell script using gemini-2.5-pro-preview-05-06 in one shot

uuid7.sh

#!/bin/bash

# Function to generate a UUID v7 string
# Implements the logic similar to libuuid's uuid_generate_time_v7
uuid_generate_time_v7() {
    local ms
    local ts_hex
    local rand_hex
    local byte6_val byte8_val
    local ver_char rand_a_part1 rand_a_part2
    local var_char rand_b_part1 rand_b_part2 rand_b_suffix

    # 1. Get 48-bit timestamp in milliseconds (unix_ts_ms)
    # GNU date specific for %N (nanoseconds)
    if date +%s%N >/dev/null 2>&1; then
        ms=$(( $(date +%s%N) / 1000000 ))
    else
        # Fallback for systems without %N (e.g., macOS default date)
        # This will have lower precision for the millisecond part
        # but is better than nothing.
        local s
        s=$(date +%s)
        # Try to get microseconds if possible (e.g. gdate on macOS)
        local us="000" # default to 000 milliseconds
        if command -v gdate >/dev/null; then # GNU date on macOS
             us=$(gdate +%N | cut -c1-3) # use first 3 digits of N for milli
        elif [[ "$(uname)" == "Linux" ]]; then
             # This case should ideally not be hit if the first date +%s%N failed
             # but as a safeguard.
             us=$(date +%N | cut -c1-3)
        fi
        ms=$(( s * 1000 + 10#$us )) # 10# to ensure base 10 for us
    fi

    # Format as 12 hex characters (6 bytes / 48 bits)
    # Ensure it's exactly 12 characters, padding with leading zeros if necessary.
    ts_hex=$(printf "%012x" "$ms")

    # 2. Get 10 random bytes (for rand_a and rand_b parts)
    # openssl is a good source of random hex. 10 bytes = 20 hex characters.
    if command -v openssl >/dev/null; then
        rand_hex=$(openssl rand -hex 10)
    else
        # Fallback if openssl is not available (less ideal, might be slower/less random quality)
        # This generates 10 bytes, then converts to hex.
        rand_hex=$(head -c 10 /dev/urandom | od -An -tx1 | tr -d ' \n' | sed 's/ //g')
        # Ensure rand_hex is 20 characters long, pad if necessary (shouldn't be with od)
        while ((${#rand_hex} < 20)); do rand_hex="0$rand_hex"; done
    fi


    # 3. Apply version (7) and variant (RFC 4122)
    # Byte 6: (rand_hex characters 0-1)
    # version = 7 (0111)
    # uuid[6] = (rand_byte_6 & 0x0F) | 0x70
    byte6_val=$(( (0x${rand_hex:0:2} & 0x0F) | 0x70 ))
    ver_char=$(printf "%x" "$(( (byte6_val & 0xF0) >> 4 ))") # Should be '7'
    rand_a_part1=$(printf "%x" "$((byte6_val & 0x0F))")     # Lower nibble of modified byte 6

    # Byte 7: (rand_hex characters 2-3)
    # This is the rest of rand_a
    rand_a_part2="${rand_hex:2:2}"

    # Byte 8: (rand_hex characters 4-5)
    # variant = 2 (10xx)
    # uuid[8] = (rand_byte_8 & 0x3F) | 0x80
    byte8_val=$(( (0x${rand_hex:4:2} & 0x3F) | 0x80 ))
    var_char=$(printf "%x" "$(( (byte8_val & 0xC0) >> 6 ))") # Should be 2 (binary 10)
                                                            # resulting hex will be 8,9,a,b
    rand_b_part1=$(printf "%x" "$((byte8_val & 0x3F))")     # Lower 6 bits of modified byte 8

    # Byte 9: (rand_hex characters 6-7)
    # This is more of rand_b
    rand_b_part2="${rand_hex:6:2}"

    # Bytes 10-15: (rand_hex characters 8-19)
    # The rest of rand_b
    rand_b_suffix="${rand_hex:8:12}"


    # 4. Assemble the UUID string
    # xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx
    # M is version, N is variant
    # Timestamp part 1 (8 hex chars)
    # Timestamp part 2 (4 hex chars)
    # Version (1 hex char from ver_char) + rand_a_part1 (1 hex char) + rand_a_part2 (2 hex chars)
    # Variant (1 hex char from first nibble of byte8_val) + rand_b_part1 (1 hex char from second nibble of byte8_val) + rand_b_part2 (2 hex chars)
    # Suffix (12 hex chars)

    printf "%s-%s-%s%s%s-%s%s%s-%s\n" \
        "${ts_hex:0:8}" \
        "${ts_hex:8:4}" \
        "$ver_char" \
        "$rand_a_part1" \
        "$rand_a_part2" \
        "$(printf "%x" "$((byte8_val >> 4 & 0xF))")" \
        "$(printf "%x" "$((byte8_val & 0xF))")" \
        "$rand_b_part2" \
        "$rand_b_suffix"
}

# Example usage:
# uuid_generate_time_v7
# for i in {1..5}; do uuid_generate_time_v7; done