#!/bin/bash
################################################################################
# Script Name: memory-pressure-exporter.sh
# Version: 1.0
# Description: Prometheus exporter for memory and swap pressure metrics.
#              Exports PSI stall information, OOM kill events, swap activity
#              rates, NUMA memory balance, slab pressure, transparent hugepage
#              stats, and zone watermark proximity.
#
# Author: Phil Connor
# Contact: contact@mylinux.work
# Website: https://mylinux.work
# License: MIT
#
# Prerequisites:
#   - Standard Unix tools (awk, grep, cat)
#   - netcat (nc) for HTTP mode
#   - Optional: journalctl (OOM tracking), kernel 4.20+ (PSI),
#     multi-node NUMA. Each section is skipped gracefully if unavailable.
#
# Usage:
#   # Output to stdout
#   ./memory-pressure-exporter.sh
#
#   # HTTP server mode
#   ./memory-pressure-exporter.sh --http -p 9198
#
#   # Textfile collector mode
#   ./memory-pressure-exporter.sh --textfile
#
# Metrics Exported:
#   Core Status:
#     - memory_pressure_up - Exporter status (1=up, 0=down)
#     - memory_pressure_exporter_info{version} - Exporter version
#
#   PSI Memory (if /proc/pressure/memory exists):
#     - memory_pressure_psi_some_avg10 - PSI memory some avg10
#     - memory_pressure_psi_some_avg60 - PSI memory some avg60
#     - memory_pressure_psi_some_avg300 - PSI memory some avg300
#     - memory_pressure_psi_some_total_microseconds - PSI memory some total
#     - memory_pressure_psi_full_avg10 - PSI memory full avg10
#     - memory_pressure_psi_full_avg60 - PSI memory full avg60
#     - memory_pressure_psi_full_avg300 - PSI memory full avg300
#     - memory_pressure_psi_full_total_microseconds - PSI memory full total
#
#   PSI I/O (if /proc/pressure/io exists):
#     - memory_pressure_psi_io_some_avg10 - PSI I/O some avg10
#     - memory_pressure_psi_io_some_avg60 - PSI I/O some avg60
#     - memory_pressure_psi_io_some_avg300 - PSI I/O some avg300
#     - memory_pressure_psi_io_some_total_microseconds - PSI I/O some total
#     - memory_pressure_psi_io_full_avg10 - PSI I/O full avg10
#     - memory_pressure_psi_io_full_avg60 - PSI I/O full avg60
#     - memory_pressure_psi_io_full_avg300 - PSI I/O full avg300
#     - memory_pressure_psi_io_full_total_microseconds - PSI I/O full total
#
#   OOM Kills (if journalctl available):
#     - memory_pressure_oom_kills_24h - OOM kills in last 24 hours
#     - memory_pressure_oom_last_kill_timestamp - Unix timestamp of last OOM
#     - memory_pressure_oom_last_victim{process} - Last killed process (1)
#
#   Swap Activity:
#     - memory_pressure_swap_in_pages_per_sec - Swap in pages/sec
#     - memory_pressure_swap_out_pages_per_sec - Swap out pages/sec
#     - memory_pressure_swap_in_bytes_per_sec - Swap in bytes/sec
#     - memory_pressure_swap_out_bytes_per_sec - Swap out bytes/sec
#
#   NUMA (if multi-node):
#     - memory_pressure_numa_total_bytes{node} - Total memory per node
#     - memory_pressure_numa_free_bytes{node} - Free memory per node
#     - memory_pressure_numa_used_percent{node} - Usage percentage per node
#
#   Transparent Hugepages:
#     - memory_pressure_thp_fault_alloc_total - THP fault allocations
#     - memory_pressure_thp_collapse_alloc_total - THP collapse allocations
#     - memory_pressure_thp_fault_fallback_total - THP fault fallbacks
#     - memory_pressure_compact_stall_total - Compaction stalls
#
#   Slab:
#     - memory_pressure_slab_reclaimable_bytes - Reclaimable slab
#     - memory_pressure_slab_unreclaimable_bytes - Unreclaimable slab
#     - memory_pressure_slab_total_bytes - Total slab
#     - memory_pressure_slab_unreclaimable_percent - Unreclaimable percentage
#
#   Zone Watermarks:
#     - memory_pressure_zone_free_pages{zone} - Current free pages
#     - memory_pressure_zone_min_pages{zone} - Min watermark
#     - memory_pressure_zone_low_pages{zone} - Low watermark
#     - memory_pressure_zone_high_pages{zone} - High watermark
#     - memory_pressure_zone_free_above_low{zone} - 1 if free > low
#
#   Exporter:
#     - memory_pressure_exporter_duration_seconds - Script execution time
#     - memory_pressure_exporter_last_run_timestamp - Last run timestamp
#
# Configuration:
#   Default HTTP port: 9198
#   Textfile directory: /var/lib/node_exporter
#   SAMPLE_INTERVAL: seconds between swap activity samples (default: 1)
#
################################################################################

# ============================================================================
# CONFIGURATION VARIABLES
# ============================================================================

TEXTFILE_DIR="/var/lib/node_exporter"
OUTPUT_FILE=""
HTTP_MODE=false
HTTP_PORT=9198
SAMPLE_INTERVAL="${SAMPLE_INTERVAL:-1}"

# ============================================================================
# HELPER FUNCTIONS
# ============================================================================

show_usage() {
    cat <<EOF
Usage: $0 [OPTIONS]

Export memory and swap pressure statistics as Prometheus metrics (v1.0).

MODES:
    --textfile      Write to node_exporter textfile collector
    --http          Run HTTP server on port $HTTP_PORT

OPTIONS:
    -p, --port      HTTP port (default: 9198)
    -o, --output    Output file path

EXAMPLES:
    $0 --textfile                    # Write to textfile collector
    $0 --http --port 9198            # Run HTTP server
    $0 -o /tmp/memory_pressure.prom  # Write to custom file

SECTIONS (auto-detected, skipped if unavailable):
    - PSI memory and I/O pressure (requires kernel 4.20+)
    - OOM kill tracking (requires journalctl)
    - Swap activity rates (always available)
    - NUMA memory balance (requires multi-node system)
    - Transparent hugepage stats (always available)
    - Slab pressure (always available)
    - Zone watermark proximity (always available)

EOF
    exit 0
}

parse_args() {
    while [[ $# -gt 0 ]]; do
        case $1 in
            -h|--help) show_usage ;;
            --textfile) OUTPUT_FILE="$TEXTFILE_DIR/memory_pressure.prom"; shift ;;
            --http) HTTP_MODE=true; shift ;;
            -p|--port) HTTP_PORT="$2"; shift 2 ;;
            -o|--output) OUTPUT_FILE="$2"; shift 2 ;;
            *) echo "Unknown option: $1" >&2; exit 1 ;;
        esac
    done
}

# ============================================================================
# PSI PRESSURE
# ============================================================================

# Parse a PSI file (/proc/pressure/memory or /proc/pressure/io)
# Args: $1 - file path
# Output: lines of "type avg10 avg60 avg300 total"
#   where type is "some" or "full"
get_psi_stats() {
    local psi_file="$1"
    [ -f "$psi_file" ] || return

    awk '{
        type = $1
        avg10 = avg60 = avg300 = total = 0
        for (i = 2; i <= NF; i++) {
            split($i, kv, "=")
            if (kv[1] == "avg10")  avg10  = kv[2]
            if (kv[1] == "avg60")  avg60  = kv[2]
            if (kv[1] == "avg300") avg300 = kv[2]
            if (kv[1] == "total")  total  = kv[2]
        }
        print type, avg10, avg60, avg300, total
    }' "$psi_file"
}

# ============================================================================
# OOM KILL TRACKING
# ============================================================================

# Get OOM kill count in last 24 hours
# Returns: count
get_oom_kill_count() {
    if ! command -v journalctl >/dev/null 2>&1; then
        echo "0"
        return
    fi
    local count
    count=$(journalctl -k --grep="Out of memory" --since "24 hours ago" --no-pager -q 2>/dev/null | wc -l)
    echo "${count:-0}"
}

# Get last OOM kill timestamp (unix epoch)
# Returns: timestamp or 0
get_oom_last_timestamp() {
    if ! command -v journalctl >/dev/null 2>&1; then
        echo "0"
        return
    fi
    local last_line
    last_line=$(journalctl -k --grep="Out of memory" --since "24 hours ago" --no-pager -q -o short-unix 2>/dev/null | tail -1)
    if [ -n "$last_line" ]; then
        echo "$last_line" | awk '{printf "%d", $1}'
    else
        echo "0"
    fi
}

# Get last OOM victim process name
# Returns: process name or empty
get_oom_last_victim() {
    if ! command -v journalctl >/dev/null 2>&1; then
        return
    fi
    journalctl -k --grep="Killed process" --since "24 hours ago" --no-pager -q 2>/dev/null \
        | tail -1 \
        | grep -oP 'Killed process \d+ \(\K[^)]+' \
        | head -1
}

# ============================================================================
# SWAP ACTIVITY
# ============================================================================

# Read swap counters from /proc/vmstat
# Returns: "pswpin pswpout"
get_swap_counters() {
    awk '/^pswpin / { pin=$2 } /^pswpout / { pout=$2 } END { print pin, pout }' /proc/vmstat 2>/dev/null
}

# ============================================================================
# NUMA MEMORY
# ============================================================================

# Check if system has multiple NUMA nodes
# Returns: 0 (true) if multi-node, 1 (false) if single
is_numa_multi_node() {
    [ -d /sys/devices/system/node/node1 ]
}

# Get NUMA memory info per node
# Output: lines of "nodeN total_kb free_kb"
get_numa_memory() {
    local node_dir="/sys/devices/system/node"
    [ -d "$node_dir" ] || return

    for node_path in "$node_dir"/node[0-9]*; do
        [ -d "$node_path" ] || continue
        local node_name
        node_name=$(basename "$node_path")
        local meminfo="$node_path/meminfo"
        [ -f "$meminfo" ] || continue

        local total free
        total=$(awk '/MemTotal/ {print $4}' "$meminfo" 2>/dev/null)
        free=$(awk '/MemFree/ {print $4}' "$meminfo" 2>/dev/null)
        echo "$node_name ${total:-0} ${free:-0}"
    done
}

# ============================================================================
# TRANSPARENT HUGEPAGES & COMPACTION
# ============================================================================

# Get THP and compaction stats from /proc/vmstat
# Returns: "thp_fault_alloc thp_collapse_alloc thp_fault_fallback compact_stall"
get_thp_stats() {
    awk '
        /^thp_fault_alloc /        { fault=$2 }
        /^thp_collapse_alloc /     { collapse=$2 }
        /^thp_fault_fallback /     { fallback=$2 }
        /^compact_stall /          { stall=$2 }
        END { print fault+0, collapse+0, fallback+0, stall+0 }
    ' /proc/vmstat 2>/dev/null
}

# ============================================================================
# SLAB MEMORY
# ============================================================================

# Get slab memory from /proc/meminfo
# Returns: "reclaimable_kb unreclaimable_kb"
get_slab_stats() {
    awk '
        /^SReclaimable:/ { reclaimable=$2 }
        /^SUnreclaim:/   { unreclaimable=$2 }
        END { print reclaimable+0, unreclaimable+0 }
    ' /proc/meminfo 2>/dev/null
}

# ============================================================================
# ZONE WATERMARKS
# ============================================================================

# Parse /proc/zoneinfo for Normal and DMA32 zones
# Output: lines of "zone free min low high"
get_zone_watermarks() {
    awk '
        /^Node [0-9]+, zone +[A-Za-z0-9]+/ {
            zone = $NF
        }
        zone == "Normal" || zone == "DMA32" {
            if ($1 == "pages" && $2 == "free") free = $3
            if ($1 == "min")  min_wm = $2
            if ($1 == "low")  low_wm = $2
            if ($1 == "high") {
                high_wm = $2
                print zone, free+0, min_wm+0, low_wm+0, high_wm+0
                zone = ""
            }
        }
    ' /proc/zoneinfo 2>/dev/null
}

# ============================================================================
# METRICS GENERATION
# ============================================================================

generate_metrics() {
    local script_start
    script_start=$(date +%s)

    # ========================================================================
    # Exporter Status
    # ========================================================================
    cat <<EOF
# HELP memory_pressure_up Exporter status (1=up)
# TYPE memory_pressure_up gauge
memory_pressure_up 1

# HELP memory_pressure_exporter_info Exporter version information
# TYPE memory_pressure_exporter_info gauge
memory_pressure_exporter_info{version="1.0"} 1

EOF

    # ========================================================================
    # PSI Memory Pressure
    # ========================================================================
    if [ -f /proc/pressure/memory ]; then
        local psi_mem
        psi_mem=$(get_psi_stats /proc/pressure/memory)

        if [ -n "$psi_mem" ]; then
            echo "$psi_mem" | while read -r type avg10 avg60 avg300 total; do
                echo "# HELP memory_pressure_psi_${type}_avg10 PSI memory ${type} avg10 percentage"
                echo "# TYPE memory_pressure_psi_${type}_avg10 gauge"
                echo "memory_pressure_psi_${type}_avg10 $avg10"
                echo ""
                echo "# HELP memory_pressure_psi_${type}_avg60 PSI memory ${type} avg60 percentage"
                echo "# TYPE memory_pressure_psi_${type}_avg60 gauge"
                echo "memory_pressure_psi_${type}_avg60 $avg60"
                echo ""
                echo "# HELP memory_pressure_psi_${type}_avg300 PSI memory ${type} avg300 percentage"
                echo "# TYPE memory_pressure_psi_${type}_avg300 gauge"
                echo "memory_pressure_psi_${type}_avg300 $avg300"
                echo ""
                echo "# HELP memory_pressure_psi_${type}_total_microseconds PSI memory ${type} total stall time in microseconds"
                echo "# TYPE memory_pressure_psi_${type}_total_microseconds counter"
                echo "memory_pressure_psi_${type}_total_microseconds $total"
                echo ""
            done
        fi
    fi

    # ========================================================================
    # PSI I/O Pressure
    # ========================================================================
    if [ -f /proc/pressure/io ]; then
        local psi_io
        psi_io=$(get_psi_stats /proc/pressure/io)

        if [ -n "$psi_io" ]; then
            echo "$psi_io" | while read -r type avg10 avg60 avg300 total; do
                echo "# HELP memory_pressure_psi_io_${type}_avg10 PSI I/O ${type} avg10 percentage"
                echo "# TYPE memory_pressure_psi_io_${type}_avg10 gauge"
                echo "memory_pressure_psi_io_${type}_avg10 $avg10"
                echo ""
                echo "# HELP memory_pressure_psi_io_${type}_avg60 PSI I/O ${type} avg60 percentage"
                echo "# TYPE memory_pressure_psi_io_${type}_avg60 gauge"
                echo "memory_pressure_psi_io_${type}_avg60 $avg60"
                echo ""
                echo "# HELP memory_pressure_psi_io_${type}_avg300 PSI I/O ${type} avg300 percentage"
                echo "# TYPE memory_pressure_psi_io_${type}_avg300 gauge"
                echo "memory_pressure_psi_io_${type}_avg300 $avg300"
                echo ""
                echo "# HELP memory_pressure_psi_io_${type}_total_microseconds PSI I/O ${type} total stall time in microseconds"
                echo "# TYPE memory_pressure_psi_io_${type}_total_microseconds counter"
                echo "memory_pressure_psi_io_${type}_total_microseconds $total"
                echo ""
            done
        fi
    fi

    # ========================================================================
    # OOM Kill Events
    # ========================================================================
    local oom_count oom_timestamp oom_victim
    oom_count=$(get_oom_kill_count)
    oom_timestamp=$(get_oom_last_timestamp)
    oom_victim=$(get_oom_last_victim)

    cat <<EOF
# HELP memory_pressure_oom_kills_24h OOM kills in the last 24 hours
# TYPE memory_pressure_oom_kills_24h gauge
memory_pressure_oom_kills_24h ${oom_count:-0}

# HELP memory_pressure_oom_last_kill_timestamp Unix timestamp of last OOM kill (0 if none)
# TYPE memory_pressure_oom_last_kill_timestamp gauge
memory_pressure_oom_last_kill_timestamp ${oom_timestamp:-0}

EOF

    if [ -n "$oom_victim" ]; then
        cat <<EOF
# HELP memory_pressure_oom_last_victim Last OOM-killed process (value is always 1)
# TYPE memory_pressure_oom_last_victim gauge
memory_pressure_oom_last_victim{process="$oom_victim"} 1

EOF
    fi

    # ========================================================================
    # Swap Activity (two-sample delta)
    # ========================================================================
    local swap1 swap2
    swap1=$(get_swap_counters)

    sleep "$SAMPLE_INTERVAL"

    swap2=$(get_swap_counters)

    local pin1 pout1 pin2 pout2
    pin1=$(echo "$swap1" | awk '{print $1}')
    pout1=$(echo "$swap1" | awk '{print $2}')
    pin2=$(echo "$swap2" | awk '{print $1}')
    pout2=$(echo "$swap2" | awk '{print $2}')

    local swap_in_rate swap_out_rate swap_in_bytes swap_out_bytes
    swap_in_rate=$(awk "BEGIN {printf \"%.2f\", (${pin2:-0} - ${pin1:-0}) / $SAMPLE_INTERVAL}")
    swap_out_rate=$(awk "BEGIN {printf \"%.2f\", (${pout2:-0} - ${pout1:-0}) / $SAMPLE_INTERVAL}")
    swap_in_bytes=$(awk "BEGIN {printf \"%.2f\", ((${pin2:-0} - ${pin1:-0}) * 4096) / $SAMPLE_INTERVAL}")
    swap_out_bytes=$(awk "BEGIN {printf \"%.2f\", ((${pout2:-0} - ${pout1:-0}) * 4096) / $SAMPLE_INTERVAL}")

    cat <<EOF
# HELP memory_pressure_swap_in_pages_per_sec Pages swapped in per second
# TYPE memory_pressure_swap_in_pages_per_sec gauge
memory_pressure_swap_in_pages_per_sec $swap_in_rate

# HELP memory_pressure_swap_out_pages_per_sec Pages swapped out per second
# TYPE memory_pressure_swap_out_pages_per_sec gauge
memory_pressure_swap_out_pages_per_sec $swap_out_rate

# HELP memory_pressure_swap_in_bytes_per_sec Bytes swapped in per second
# TYPE memory_pressure_swap_in_bytes_per_sec gauge
memory_pressure_swap_in_bytes_per_sec $swap_in_bytes

# HELP memory_pressure_swap_out_bytes_per_sec Bytes swapped out per second
# TYPE memory_pressure_swap_out_bytes_per_sec gauge
memory_pressure_swap_out_bytes_per_sec $swap_out_bytes

EOF

    # ========================================================================
    # NUMA Memory Balance
    # ========================================================================
    if is_numa_multi_node; then
        local numa_data
        numa_data=$(get_numa_memory)

        if [ -n "$numa_data" ]; then
            echo "$numa_data" | while read -r node total_kb free_kb; do
                local total_bytes free_bytes used_pct
                total_bytes=$((total_kb * 1024))
                free_bytes=$((free_kb * 1024))
                if [ "$total_kb" -gt 0 ]; then
                    used_pct=$(awk "BEGIN {printf \"%.2f\", (($total_kb - $free_kb) / $total_kb) * 100}")
                else
                    used_pct="0.00"
                fi
                echo "# HELP memory_pressure_numa_total_bytes Total memory per NUMA node in bytes"
                echo "# TYPE memory_pressure_numa_total_bytes gauge"
                echo "memory_pressure_numa_total_bytes{node=\"$node\"} $total_bytes"
                echo ""
                echo "# HELP memory_pressure_numa_free_bytes Free memory per NUMA node in bytes"
                echo "# TYPE memory_pressure_numa_free_bytes gauge"
                echo "memory_pressure_numa_free_bytes{node=\"$node\"} $free_bytes"
                echo ""
                echo "# HELP memory_pressure_numa_used_percent Memory usage percentage per NUMA node"
                echo "# TYPE memory_pressure_numa_used_percent gauge"
                echo "memory_pressure_numa_used_percent{node=\"$node\"} $used_pct"
                echo ""
            done
        fi
    fi

    # ========================================================================
    # Transparent Hugepages & Compaction
    # ========================================================================
    local thp_stats
    thp_stats=$(get_thp_stats)

    if [ -n "$thp_stats" ]; then
        local thp_fault thp_collapse thp_fallback compact_stall
        thp_fault=$(echo "$thp_stats" | awk '{print $1}')
        thp_collapse=$(echo "$thp_stats" | awk '{print $2}')
        thp_fallback=$(echo "$thp_stats" | awk '{print $3}')
        compact_stall=$(echo "$thp_stats" | awk '{print $4}')

        cat <<EOF
# HELP memory_pressure_thp_fault_alloc_total THP fault allocations
# TYPE memory_pressure_thp_fault_alloc_total counter
memory_pressure_thp_fault_alloc_total ${thp_fault:-0}

# HELP memory_pressure_thp_collapse_alloc_total THP collapse allocations
# TYPE memory_pressure_thp_collapse_alloc_total counter
memory_pressure_thp_collapse_alloc_total ${thp_collapse:-0}

# HELP memory_pressure_thp_fault_fallback_total THP fault fallbacks to regular pages
# TYPE memory_pressure_thp_fault_fallback_total counter
memory_pressure_thp_fault_fallback_total ${thp_fallback:-0}

# HELP memory_pressure_compact_stall_total Memory compaction stalls
# TYPE memory_pressure_compact_stall_total counter
memory_pressure_compact_stall_total ${compact_stall:-0}

EOF
    fi

    # ========================================================================
    # Slab Memory
    # ========================================================================
    local slab_stats
    slab_stats=$(get_slab_stats)

    if [ -n "$slab_stats" ]; then
        local slab_reclaim_kb slab_unreclaim_kb
        slab_reclaim_kb=$(echo "$slab_stats" | awk '{print $1}')
        slab_unreclaim_kb=$(echo "$slab_stats" | awk '{print $2}')

        local slab_reclaim_bytes slab_unreclaim_bytes slab_total_bytes slab_unreclaim_pct
        slab_reclaim_bytes=$((slab_reclaim_kb * 1024))
        slab_unreclaim_bytes=$((slab_unreclaim_kb * 1024))
        slab_total_bytes=$(( slab_reclaim_bytes + slab_unreclaim_bytes ))

        if [ "$slab_total_bytes" -gt 0 ]; then
            slab_unreclaim_pct=$(awk "BEGIN {printf \"%.2f\", ($slab_unreclaim_bytes / $slab_total_bytes) * 100}")
        else
            slab_unreclaim_pct="0.00"
        fi

        cat <<EOF
# HELP memory_pressure_slab_reclaimable_bytes Reclaimable slab memory in bytes
# TYPE memory_pressure_slab_reclaimable_bytes gauge
memory_pressure_slab_reclaimable_bytes $slab_reclaim_bytes

# HELP memory_pressure_slab_unreclaimable_bytes Unreclaimable slab memory in bytes
# TYPE memory_pressure_slab_unreclaimable_bytes gauge
memory_pressure_slab_unreclaimable_bytes $slab_unreclaim_bytes

# HELP memory_pressure_slab_total_bytes Total slab memory in bytes
# TYPE memory_pressure_slab_total_bytes gauge
memory_pressure_slab_total_bytes $slab_total_bytes

# HELP memory_pressure_slab_unreclaimable_percent Percentage of slab memory that is unreclaimable
# TYPE memory_pressure_slab_unreclaimable_percent gauge
memory_pressure_slab_unreclaimable_percent $slab_unreclaim_pct

EOF
    fi

    # ========================================================================
    # Zone Watermarks
    # ========================================================================
    local zone_data
    zone_data=$(get_zone_watermarks)

    if [ -n "$zone_data" ]; then
        echo "$zone_data" | while read -r zone free min_wm low_wm high_wm; do
            local above_low=1
            if [ "$free" -le "$low_wm" ]; then
                above_low=0
            fi
            echo "# HELP memory_pressure_zone_free_pages Current free pages per zone"
            echo "# TYPE memory_pressure_zone_free_pages gauge"
            echo "memory_pressure_zone_free_pages{zone=\"$zone\"} $free"
            echo ""
            echo "# HELP memory_pressure_zone_min_pages Min watermark pages per zone"
            echo "# TYPE memory_pressure_zone_min_pages gauge"
            echo "memory_pressure_zone_min_pages{zone=\"$zone\"} $min_wm"
            echo ""
            echo "# HELP memory_pressure_zone_low_pages Low watermark pages per zone"
            echo "# TYPE memory_pressure_zone_low_pages gauge"
            echo "memory_pressure_zone_low_pages{zone=\"$zone\"} $low_wm"
            echo ""
            echo "# HELP memory_pressure_zone_high_pages High watermark pages per zone"
            echo "# TYPE memory_pressure_zone_high_pages gauge"
            echo "memory_pressure_zone_high_pages{zone=\"$zone\"} $high_wm"
            echo ""
            echo "# HELP memory_pressure_zone_free_above_low Whether free pages are above the low watermark (1=above, 0=below)"
            echo "# TYPE memory_pressure_zone_free_above_low gauge"
            echo "memory_pressure_zone_free_above_low{zone=\"$zone\"} $above_low"
            echo ""
        done
    fi

    # ========================================================================
    # Exporter Runtime
    # ========================================================================
    local script_end script_duration
    script_end=$(date +%s)
    script_duration=$((script_end - script_start))

    cat <<EOF
# HELP memory_pressure_exporter_duration_seconds Time to generate all metrics
# TYPE memory_pressure_exporter_duration_seconds gauge
memory_pressure_exporter_duration_seconds $script_duration

# HELP memory_pressure_exporter_last_run_timestamp Unix timestamp of last successful run
# TYPE memory_pressure_exporter_last_run_timestamp gauge
memory_pressure_exporter_last_run_timestamp $script_end
EOF

    echo ""
}

# ============================================================================
# HTTP SERVER MODE
# ============================================================================

run_http_server() {
    echo "Starting memory pressure exporter on port $HTTP_PORT..." >&2

    if ! command -v nc >/dev/null 2>&1; then
        echo "ERROR: netcat (nc) required for HTTP mode" >&2
        exit 1
    fi

    while true; do
        {
            read -r request
            if [[ "$request" =~ ^GET\ /metrics ]]; then
                echo -e "HTTP/1.1 200 OK\r\nContent-Type: text/plain; version=0.0.4\r\n\r"
                generate_metrics
            else
                echo -e "HTTP/1.1 200 OK\r\nContent-Type: text/html\r\n\r"
                cat <<EOF
<!DOCTYPE html>
<html>
<head><title>Memory Pressure Exporter v1.0</title></head>
<body>
<h1>Memory Pressure Exporter v1.0</h1>
<p><a href="/metrics">Metrics</a></p>
<h2>Sections (auto-detected)</h2>
<ul>
<li>PSI memory and I/O pressure (requires kernel 4.20+)</li>
<li>OOM kill tracking (requires journalctl)</li>
<li>Swap activity rates</li>
<li>NUMA memory balance (requires multi-node system)</li>
<li>Transparent hugepage and compaction stats</li>
<li>Slab memory pressure</li>
<li>Zone watermark proximity</li>
</ul>
</body>
</html>
EOF
            fi
        } | nc -l -p "$HTTP_PORT" -q 1 2>/dev/null
    done
}

# ============================================================================
# MAIN EXECUTION
# ============================================================================

main() {
    parse_args "$@"

    if [ "$HTTP_MODE" = true ]; then
        run_http_server
    elif [ -n "$OUTPUT_FILE" ]; then
        local output_dir
        output_dir="$(dirname "$OUTPUT_FILE")"
        mkdir -p "$output_dir"

        local temp_file
        temp_file=$(mktemp "${output_dir}/.memory_pressure_metrics.XXXXXX")

        if ! generate_metrics > "$temp_file" 2>/dev/null; then
            rm -f "$temp_file"
            echo "ERROR: Failed to generate metrics" >&2
            exit 1
        fi

        local file_lines
        file_lines=$(wc -l < "$temp_file" 2>/dev/null || echo 0)

        if [ "$file_lines" -lt 10 ]; then
            rm -f "$temp_file"
            echo "ERROR: Metrics file too small ($file_lines lines), keeping previous" >&2
            exit 1
        fi

        chmod 644 "$temp_file"
        mv -f "$temp_file" "$OUTPUT_FILE"

        echo "Metrics written to $OUTPUT_FILE ($file_lines lines)" >&2
    else
        generate_metrics
    fi
}

main "$@"