#!/bin/sh
set -eu

INPUT="${1:?input file required}"
OUTPUT="${2:?output file required}"

mkdir -p "$(dirname "$OUTPUT")"

TMP="$(mktemp)"
BASE_CREATED=0

if [ -f "$OUTPUT" ]; then
    BASE="$OUTPUT"
else
    BASE="$(mktemp)"
    BASE_CREATED=1
    : > "$BASE"
fi

cleanup() {
    rm -f "$TMP"
    if [ "$BASE_CREATED" = "1" ]; then
        rm -f "$BASE"
    fi
}
trap cleanup EXIT INT TERM

awk '
function trim(s) {
    sub(/^[[:space:]]+/, "", s)
    sub(/[[:space:]]+$/, "", s)
    return s
}

function parse_key(line,    eq, key) {
    eq = index(line, "=")
    if (eq == 0) {
        return ""
    }

    key = trim(substr(line, 1, eq - 1))

    if (key !~ /^[A-Za-z_][A-Za-z0-9_]*$/) {
        return ""
    }

    return key
}

function merged_line(key, line) {
    if (key ~ /^MKS_/ && key in ENVIRON) {
        return key "=" ENVIRON[key]
    }

    return line
}

# First file: INPUT
phase == 1 {
    line = $0
    key = parse_key(line)

    if (key != "") {
        incoming[key] = merged_line(key, line)

        if (!(key in input_seen)) {
            input_order[++input_count] = key
            input_seen[key] = 1
        }
    }

    next
}

# Second file: existing OUTPUT / BASE
phase == 2 {
    line = $0
    key = parse_key(line)

    if (key != "" && key in incoming) {
        print incoming[key]
        written[key] = 1
        next
    }

    print line

    if (key != "") {
        written[key] = 1
    }

    next
}

END {
    for (i = 1; i <= input_count; i++) {
        key = input_order[i]

        if (!(key in written)) {
            print incoming[key]
            written[key] = 1
        }
    }
}
' phase=1 "$INPUT" phase=2 "$BASE" > "$TMP"

mv "$TMP" "$OUTPUT"