#!/usr/bin/env bash

# External monitor brightness over DDC/CI.
#
# A desktop with no backlight class device has no brightness control at all --
# brightnessctl only sees keyboard and NIC LEDs. The panel itself still has a
# brightness setting, reachable over the monitor's DDC/CI channel (VCP feature
# 0x10), which is what the buttons on the bezel drive.
#
# Usage:
#   panama-brightness list            -> {"displays":[...],"error":""}
#   panama-brightness get <bus>       -> integer percent
#   panama-brightness set <bus> <pct> -> applies, prints nothing
#
# Displays are enumerated from sysfs rather than from `ddcutil detect`. The
# kernel publishes the connector-to-I2C-bus mapping directly, as
# /sys/class/drm/<card>-<connector>/ddc, along with whether anything is plugged
# in. That is better than parsing detect output in three ways: the format is
# stable where detect's brief output is undocumented, the connector name comes
# out exactly as Hyprland and the Displays page already spell it (DP-2), and
# only connectors with a monitor attached get probed -- one bus on this machine
# instead of fourteen, which is the difference between a fast scan and a slow
# one, since each probe of an empty bus waits for a timeout.
#
# No model name is reported. Hyprland already knows the human-readable
# description of every output, so the UI joins on the connector name rather than
# having two sources of truth for what a monitor is called.

set -uo pipefail

readonly VCP_BRIGHTNESS=0x10

# Test seams. The contract needs to exercise enumeration and parsing on a
# machine whose real monitors it must not touch, so both roots this script
# reads are overridable. Nothing sets them in normal use.
readonly DRM_ROOT="${PANAMA_BRIGHTNESS_DRM_ROOT:-/sys/class/drm}"
readonly DEV_ROOT="${PANAMA_BRIGHTNESS_DEV_ROOT:-/dev}"

emit_error() {
    printf '{"displays":[],"error":%s}\n' "$(jq -Rn --arg e "$1" '$e')"
    exit 0
}

command -v ddcutil >/dev/null 2>&1 || emit_error 'ddcutil is not installed'

# Reading a VCP value needs read/write access to the monitor's I2C bus. The
# udev rule ddcutil ships grants that to the seat user through uaccess, but only
# to devices created after the rule was installed -- so a machine that installed
# ddcutil without rebooting has the rule in place and no access to show for it.
# That is by far the most likely reason for an empty list, and it is fixable in
# one command, so say so rather than reporting "no displays".
has_accessible_bus() {
    local dev
    for dev in "$DEV_ROOT"/i2c-*; do
        [[ -r "$dev" && -w "$dev" ]] && return 0
    done
    return 1
}

cmd_list() {
    has_accessible_bus || emit_error 'no I2C bus is accessible. ddcutil ships a udev rule that grants this, but only to devices created after it was installed. Run: sudo udevadm control --reload-rules && sudo udevadm trigger --subsystem-match=i2c-dev --subsystem-match=drm'

    local rows=() connector bus value path
    for path in "$DRM_ROOT"/card*-*; do
        [[ -e "$path/ddc" ]] || continue
        [[ "$(cat "$path/status" 2>/dev/null)" == "connected" ]] || continue

        # card1-DP-2 -> DP-2, the name Hyprland uses.
        connector="$(basename "$path")"
        connector="${connector#card*-}"

        bus="$(basename "$(readlink -f "$path/ddc")")"
        bus="${bus#i2c-}"
        [[ "$bus" =~ ^[0-9]+$ ]] || continue

        # A monitor that does not implement 0x10 is not an error; it simply
        # cannot be controlled, and is left out rather than shown as a slider
        # that does nothing.
        value="$(cmd_get "$bus")" || continue
        [[ -n "$value" ]] || continue

        rows+=("$(jq -cn \
            --argjson bus "$bus" \
            --arg connector "$connector" \
            --argjson value "$value" \
            '{bus: $bus, connector: $connector, value: $value}')")
    done

    if [[ ${#rows[@]} -eq 0 ]]; then
        emit_error 'no connected monitor reports DDC/CI brightness. Some panels implement it only when "DDC/CI" is enabled in their on-screen menu.'
    fi

    printf '{"displays":[%s],"error":""}\n' "$(IFS=,; printf '%s' "${rows[*]}")"
}

# Prints the current brightness as a whole percent, or nothing when the display
# cannot report it. `getvcp --brief` is documented as machine readable and
# answers "VCP 10 C <current> <max>"; the max is almost always 100 but is not
# guaranteed to be, so it is read rather than assumed.
cmd_get() {
    local bus="$1" out current max
    out="$(timeout 10 ddcutil --bus "$bus" getvcp "$VCP_BRIGHTNESS" --brief 2>/dev/null)" || return 1
    read -r _ _ _ current max <<<"$out"
    [[ "$current" =~ ^[0-9]+$ && "$max" =~ ^[0-9]+$ && "$max" -gt 0 ]] || return 1
    printf '%s' "$(( current * 100 / max ))"
}

cmd_set() {
    local bus="$1" percent="$2" max out
    [[ "$percent" =~ ^[0-9]+$ ]] || return 1
    (( percent > 100 )) && percent=100

    out="$(timeout 10 ddcutil --bus "$bus" getvcp "$VCP_BRIGHTNESS" --brief 2>/dev/null)" || return 1
    read -r _ _ _ _ max <<<"$out"
    [[ "$max" =~ ^[0-9]+$ && "$max" -gt 0 ]] || max=100

    timeout 10 ddcutil --bus "$bus" setvcp "$VCP_BRIGHTNESS" "$(( percent * max / 100 ))" >/dev/null 2>&1
}

case "${1:-list}" in
    list) cmd_list ;;
    get)  cmd_get "${2:?bus required}" ;;
    set)  cmd_set "${2:?bus required}" "${3:?percent required}" ;;
    *)    printf 'usage: panama-brightness [list|get <bus>|set <bus> <percent>]\n' >&2; exit 2 ;;
esac