#!/usr/bin/env bash

if [[ -z "$1" ]]; then
    cat << EOF
reldate - Format or adjust dates

Usage: reldate <date|adjustment|keyword>

Examples:
  reldate "2025-12-12"    # Output: 2025-12-12
  reldate "+1d"           # Output: tomorrow's date
  reldate "-1w"           # Output: date 1 week ago
  reldate "+2m"           # Output: date 2 months from now
  reldate "-1y"           # Output: date 1 year ago

  reldate today           # Output: today's date
  reldate tomorrow        # Output: tomorrow's date
  reldate yesterday       # Output: yesterday's date

  reldate friday          # Output: next Friday
  reldate fri             # Output: next Friday
  reldate weekend         # Output: next Saturday

Adjustments:
  d = days, w = weeks, m = months, y = years
  Use + for future, - for past

Keywords:
  today, tomorrow, yesterday
  monday-sunday (or mon-sun)
  weekend (Saturday)

EOF
    exit 1
fi

# Normalize input to lowercase
input=$(echo "$1" | tr '[:upper:]' '[:lower:]')

# Calculate days until a target weekday (1=Mon, 7=Sun)
# Returns days to add (1-7, never 0 - same day means next week)
days_until_weekday() {
    local target=$1
    local current=$(date +%u)
    local diff=$(( (target - current + 7) % 7 ))
    if [[ $diff -eq 0 ]]; then
        diff=7
    fi
    echo $diff
}

# Map day name to number (1=Mon, 7=Sun)
day_to_number() {
    case "$1" in
        mon|monday)    echo 1 ;;
        tue|tuesday)   echo 2 ;;
        wed|wednesday) echo 3 ;;
        thu|thursday)  echo 4 ;;
        fri|friday)    echo 5 ;;
        sat|saturday)  echo 6 ;;
        sun|sunday)    echo 7 ;;
        *)             echo 0 ;;
    esac
}

# Handle natural language keywords
case "$input" in
    today)
        date "+%Y-%m-%d"
        ;;
    tomorrow)
        date -v "+1d" "+%Y-%m-%d"
        ;;
    yesterday)
        date -v "-1d" "+%Y-%m-%d"
        ;;
    weekend)
        days=$(days_until_weekday 6)
        date -v "+${days}d" "+%Y-%m-%d"
        ;;
    mon|monday|tue|tuesday|wed|wednesday|thu|thursday|fri|friday|sat|saturday|sun|sunday)
        day_num=$(day_to_number "$input")
        days=$(days_until_weekday "$day_num")
        date -v "+${days}d" "+%Y-%m-%d"
        ;;
    *)
        # Fall through to original logic
        if [[ "$1" =~ ^[0-9]{4}-[0-9]{2}-[0-9]{2}$ ]]; then
            date -j -f "%Y-%m-%d" "$1" "+%Y-%m-%d" 2>/dev/null || echo "Invalid date: $1"
        else
            date -v "$1" "+%Y-%m-%d"
        fi
        ;;
esac