#!/usr/bin/env bash
set -euo pipefail

# Download dataset files from the Turing Motors Open Dataset API.
#
# Features:
#   - Parallel downloads with configurable concurrency
#   - Byte-level progress bar with speed and ETA
#   - Resume support: re-run to continue interrupted downloads
#   - Automatic pagination for large file lists
#   - Post-download size verification
#
# Requirements: bash 4.3+, curl, awk, and standard POSIX utilities
#   (grep, sed, paste, find, du, stat, mktemp, wc)
#
# Usage:
#   ./download-dataset.sh -u <list-url> [-o <output-dir>] [-p <parallel>]

# ============================================================
# Utilities
# ============================================================

die() { echo "Error: $*" >&2; exit 1; }

usage() {
    cat <<'HELP'
Usage: download-dataset.sh -u <list-url> [-o <output-dir>] [-p <parallel>]

Options:
    -u  List API URL (required)
    -o  Output directory (default: current directory)
    -p  Number of parallel downloads (default: 8)
    -h  Show this help

Examples:
    ./download-dataset.sh -u "https://open-dataset.turing-motors.net/api/list?token=xxx"
    ./download-dataset.sh -u "https://...?token=xxx" -o ./dataset -p 16
HELP
    exit "${1:-0}"
}

# Format seconds as human-readable duration
format_duration() {
    local s=$1
    if [ "$s" -ge 3600 ]; then
        printf "%dh%02dm%02ds" $((s / 3600)) $((s % 3600 / 60)) $((s % 60))
    elif [ "$s" -ge 60 ]; then
        printf "%dm%02ds" $((s / 60)) $((s % 60))
    else
        printf "%ds" "$s"
    fi
}

# Format bytes as human-readable size (pure bash)
format_bytes() {
    local b=$1 unit=0 int frac
    local units=(B KiB MiB GiB TiB)

    int=$b
    while [ "$int" -ge 1024 ] && [ "$unit" -lt 4 ]; do
        frac=$(( (int % 1024) * 10 / 1024 ))
        int=$((int / 1024))
        unit=$((unit + 1))
    done

    if [ "$unit" -eq 0 ]; then
        printf "%d B" "$b"
    else
        printf "%d.%d %s" "$int" "$frac" "${units[$unit]}"
    fi
}

# Cross-platform file size in bytes (GNU stat / BSD stat)
file_size() {
    stat --printf="%s" "$1" 2>/dev/null && return
    stat -f %z "$1" 2>/dev/null && return
    echo 0
}

# Cross-platform directory size in bytes
dir_size_bytes() {
    local val
    # GNU du -sb: byte-precise apparent size (Linux)
    if read -r val _ < <(du -sb "$1" 2>/dev/null); then
        echo "$val"
        return
    fi
    # Fallback: du -sk in KB (macOS / BSD)
    if read -r val _ < <(du -sk "$1" 2>/dev/null); then
        echo $((val * 1024))
        return
    fi
    echo 0
}

# ============================================================
# JSON parsing (pure shell — no jq dependency)
# ============================================================

# Extract objects as tab-separated lines: key\tsize\turl
parse_objects() {
    grep -oE '"key":"[^"]*"|"size":[0-9]*|"url":"[^"]*"' \
        | paste - - - \
        | sed 's/"key":"//; s/"\t"size":/\t/; s/\t"url":"/\t/; s/"$//'
}

# Extract pagination cursor (empty string if absent)
parse_cursor() {
    grep -oE '"cursor":"[^"]*"' | sed 's/"cursor":"//; s/"$//' || true
}

# ============================================================
# Fetch file list (with automatic pagination)
# ============================================================

fetch_file_list() {
    local url=$1 response file_list cursor page

    echo "Fetching file list..." >&2
    response=$(curl -sf "$url") || die "Failed to fetch file list from API"

    file_list=$(echo "$response" | parse_objects)
    cursor=$(echo "$response" | parse_cursor)

    while [ -n "$cursor" ]; do
        echo "  fetching next page..." >&2
        page=$(curl -sf "${url}&cursor=${cursor}") \
            || die "Failed to fetch page (cursor: ${cursor:0:20}...)"
        file_list="${file_list}
$(echo "$page" | parse_objects)"
        cursor=$(echo "$page" | parse_cursor)
    done

    echo "$file_list"
}

# ============================================================
# Progress monitor (runs in background)
# ============================================================

# Build a progress bar string: [####..........]
build_bar() {
    local filled=$1 empty=$2 bar="" spc=""
    if [ "$filled" -gt 0 ]; then bar=$(printf '%*s' "$filled" '' | tr ' ' '#'); fi
    if [ "$empty" -gt 0 ];  then spc=$(printf '%*s' "$empty" ''  | tr ' ' '.'); fi
    printf "[%s%s]" "$bar" "$spc"
}

# Render a single progress line to stderr
render_progress() {
    local cur_bytes=$1 total_bytes=$2 done_count=$3 total=$4
    local active=$5 speed=$6 elapsed=$7

    local bar_w=30

    # Completion
    if [ "$done_count" -ge "$total" ] && [ "$total" -gt 0 ]; then
        printf "\r%s %s/%s  %d/%d files  elapsed: %s  done!%*s\n" \
            "$(build_bar "$bar_w" 0)" \
            "$(format_bytes "$cur_bytes")" "$(format_bytes "$total_bytes")" \
            "$done_count" "$total" "$(format_duration "$elapsed")" 20 "" >&2
        return 1  # signal: stop the monitor
    fi

    # In progress
    if [ "$cur_bytes" -gt 0 ] && [ "$total_bytes" -gt 0 ]; then
        local filled=$((cur_bytes * bar_w / total_bytes))
        if [ "$filled" -gt "$bar_w" ]; then filled=$bar_w; fi
        local empty=$((bar_w - filled))

        local remaining=$((total_bytes - cur_bytes)) eta=0
        if [ "$speed" -gt 0 ]; then eta=$((remaining / speed)); fi

        local status="${done_count}/${total} done"
        if [ "$active" -gt 0 ]; then status="${status}, ${active} active"; fi

        printf "\r%s %s/%s  %s  %s/s  ETA: %s%*s" \
            "$(build_bar "$filled" "$empty")" \
            "$(format_bytes "$cur_bytes")" "$(format_bytes "$total_bytes")" \
            "$status" "$(format_bytes "$speed")" \
            "$(format_duration "$eta")" 10 "" >&2
        return 0
    fi

    # Not started
    printf "\r%s 0/%s  elapsed: %s  starting...%*s" \
        "$(build_bar 0 "$bar_w")" \
        "$(format_bytes "$total_bytes")" \
        "$(format_duration "$elapsed")" 10 "" >&2
}

run_progress_monitor() {
    # Disable strict mode: the monitor must not die on transient errors
    set +eu

    local total=$1 total_bytes=$2 output_dir=$3 progress_dir=$4
    local max_parallel=$5 start_time=$6

    local speed_smooth=0 prev_time=$start_time
    local baseline_bytes prev_bytes
    baseline_bytes=$(dir_size_bytes "$output_dir")
    prev_bytes=0

    while true; do
        local now elapsed cur_bytes
        now=$(date +%s)
        elapsed=$((now - start_time))
        cur_bytes=$(( $(dir_size_bytes "$output_dir") - baseline_bytes ))

        # Speed: exponential moving average (70/30 smoothing)
        local dt=$((now - prev_time))
        if [ "$dt" -gt 0 ]; then
            local instant=$(( (cur_bytes - prev_bytes) / dt ))
            if [ "$instant" -lt 0 ]; then instant=0; fi
            speed_smooth=$(( (speed_smooth * 7 + instant * 3) / 10 ))
            prev_bytes=$cur_bytes
            prev_time=$now
        fi

        # Count completed and failed (single find, split by name)
        local all_count fail_count done_count
        all_count=$(find "$progress_dir" -maxdepth 1 -type f 2>/dev/null | wc -l)
        fail_count=$(find "$progress_dir" -maxdepth 1 -name '*.fail' -type f 2>/dev/null | wc -l)
        done_count=$((all_count - fail_count))

        # Estimate active downloads: capped at max_parallel
        local dispatched=$((done_count + fail_count))
        local active=$((total - dispatched))
        if [ "$active" -gt "$max_parallel" ]; then active=$max_parallel; fi
        if [ "$active" -lt 0 ]; then active=0; fi

        render_progress "$cur_bytes" "$total_bytes" "$done_count" "$total" \
            "$active" "$speed_smooth" "$elapsed" || break

        sleep 1
    done
}

# ============================================================
# Download a single file
# ============================================================

download_file() {
    local key=$1 size=$2 url=$3 dest=$4

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

    # Skip if already downloaded with correct size
    if [ -f "$dest" ] && [ "$(file_size "$dest")" = "$size" ]; then
        return 0
    fi

    # Download with resume support
    curl -fSL -C - -o "$dest" "$url" 2>/dev/null || return 1

    # Verify downloaded size
    local actual
    actual=$(file_size "$dest")
    if [ "$actual" != "$size" ]; then
        rm -f "$dest"
        return 1
    fi
}

# ============================================================
# Argument parsing
# ============================================================

parse_args() {
    list_url=""
    output_dir="."
    parallel=8

    while getopts "u:o:p:h" opt; do
        case "$opt" in
            u) list_url="$OPTARG" ;;
            o) output_dir="$OPTARG" ;;
            p) parallel="$OPTARG" ;;
            h) usage 0 ;;
            *) usage 1 >&2 ;;
        esac
    done

    if [ -z "$list_url" ]; then
        die "-u <list-url> is required"
    fi
    if ! [[ "$parallel" =~ ^[1-9][0-9]*$ ]]; then
        die "-p must be a positive integer (got: '$parallel')"
    fi
}

# ============================================================
# Cleanup
# ============================================================

monitor_pid=""
listfile=""
progress_dir=""

cleanup() {
    if [ -n "$monitor_pid" ]; then kill "$monitor_pid" 2>/dev/null || true; fi
    local pids
    pids=$(jobs -p 2>/dev/null) || true
    # shellcheck disable=SC2086  # word splitting intentional for multiple PIDs
    if [ -n "$pids" ]; then kill $pids 2>/dev/null || true; fi
    if [ -n "$listfile" ];     then rm -f "$listfile"; fi
    if [ -n "$progress_dir" ]; then rm -rf "$progress_dir"; fi
}
# EXIT handles normal exit; INT/TERM must explicitly exit after cleanup
trap cleanup EXIT
trap 'cleanup; echo ""; exit 130' INT
trap 'cleanup; exit 143' TERM

# ============================================================
# Summary
# ============================================================

print_summary() {
    local progress_dir=$1 total=$2

    local failed
    failed=$(find "$progress_dir" -maxdepth 1 -name '*.fail' -type f 2>/dev/null | wc -l)

    echo ""
    echo "Complete: $((total - failed))/$total succeeded."
    if [ "$failed" -gt 0 ]; then
        echo "Failed: $failed file(s). Re-run to retry." >&2
    fi
    return "$failed"
}

# ============================================================
# Main
# ============================================================

# Fetch file list and write to a temp file (key\tsize\turl per line).
# Sets: listfile, total, total_bytes
prepare_file_list() {
    local file_list
    file_list=$(fetch_file_list "$list_url")

    listfile=$(mktemp)
    echo "$file_list" | grep $'\t' > "$listfile"

    total=$(wc -l < "$listfile")
    total_bytes=$(awk -F'\t' '{s += $2} END {print s + 0}' "$listfile")
}

# Start the background progress monitor.
# Sets: monitor_pid
start_monitor() {
    start_time=$(date +%s)

    run_progress_monitor "$total" "$total_bytes" "$output_dir" "$progress_dir" \
        "$parallel" "$start_time" &
    monitor_pid=$!
}

# Stop the background progress monitor.
stop_monitor() {
    sleep 2
    kill "$monitor_pid" 2>/dev/null || true
    wait "$monitor_pid" 2>/dev/null || true
    monitor_pid=""
}

# Download all files using a parallel job pool (wait -n).
run_downloads() {
    local running=0 idx=0
    while IFS=$'\t' read -r key size url; do
        if [ -z "$key" ]; then continue; fi
        idx=$((idx + 1))

        (
            if download_file "$key" "$size" "$url" "$output_dir/$key"; then
                touch "$progress_dir/$idx"
            else
                touch "$progress_dir/$idx.fail"
            fi
        ) &
        running=$((running + 1))

        if [ "$running" -ge "$parallel" ]; then
            wait -n 2>/dev/null || true
            running=$((running - 1))
        fi
    done < "$listfile"

    wait 2>/dev/null || true
}

main() {
    parse_args "$@"
    mkdir -p "$output_dir"

    prepare_file_list
    progress_dir=$(mktemp -d)

    echo "Found $total file(s) to download ($(format_bytes "$total_bytes"), parallel: $parallel)."
    echo ""

    start_monitor
    run_downloads
    stop_monitor

    print_summary "$progress_dir" "$total"
}

main "$@"