#!/bin/bash
# inflate - adjust a historical USD amount for inflation using BLS CPI data
#
# Finds the equivalent buying power today* of a historical amount of USD
# e.g. "$1234.56 in Feb 1980 is equivalent to $4915.94 in Jun 2024"
# Data source: https://data.bls.gov/cgi-bin/cpicalc.pl
# * Most recent month available is used, typically a 1-month delay
#
# Usage:
#   inflate <amount> <year> [month]
#   (where month is an integer in [1,12], default 1)
#
# Sample output with stderr:
#   $ inflate 150 1970 9
#   $150.00: Sep 1970 -> Jun 2024
#   1202.20
# Sample output without stderr:
#   $ inflate 150 1970 9 2>/dev/null
#   1202.20

_inflate() (
    local SCRIPT_NAME; SCRIPT_NAME="$(basename "${BASH_SOURCE[0]}")"
    case "${BASH_SOURCE[0]}" in /dev/*|/proc/*) SCRIPT_NAME="" ;; esac
    case "$SCRIPT_NAME" in ""|bash|sh|zsh|dash) SCRIPT_NAME="inflate" ;; esac

    _error() { echo "[ERR][$SCRIPT_NAME] $*" >&2; }

    _show_help() {
        local s; [ -t 1 ] && s=$'\033[4m'
        local r; [ -t 1 ] && r=$'\033[24m'
        echo "NAME"
        echo "  $SCRIPT_NAME - adjust a historical USD amount for inflation"
        echo "SYNOPSIS"
        echo "  $SCRIPT_NAME ${s}amount${r} ${s}year${r} [${s}month${r}]"
        echo "  $SCRIPT_NAME -h"
        echo "DESCRIPTION"
        echo "  Fetches CPI data from the US Bureau of Labor Statistics and prints"
        echo "  the equivalent buying power in the most recent month available."
        echo "  The adjusted amount is printed to stdout; a summary line (e.g."
        echo "  \"\$150.00: Sep 1970 -> Jun 2024\") is printed to stderr."
        echo "  BLS CPI data is available for years 1913 through the most recent"
        echo "  month (typically a 1-2 month delay)."
        echo "OPTIONS"
        echo "  ${s}amount${r}           Dollar amount to adjust (e.g. 150, 1234.56)"
        echo "  ${s}year${r}             Historical year (1913 or later)"
        echo "  ${s}month${r}            Month as integer 1-12 (default: 1 / January)"
        echo "  -h, --help         Show this help message"
        echo "EXIT STATUS"
        echo "  0  Success"
        echo "  1  Runtime failure (BLS calculator response didn't yield a number)"
        echo "  2  Usage error (missing amount, missing year, month out of range)"
        echo "  3  Dependency error (curl missing)"
        echo "DEPENDENCIES"
        echo "  curl"
        echo "CAVEATS"
        echo "  Requires network connectivity; the BLS inflation calculator at"
        echo "  https://data.bls.gov/cgi-bin/cpicalc.pl must be reachable. Data is"
        echo "  fetched on every invocation (no local cache)."
    }

    case "$1" in
        -h|--help) _show_help; return 0 ;;
    esac

    if ! command -v curl >/dev/null 2>&1; then
        _error "curl is required"
        return 3
    fi

    [ "$1" ] || { _error "amount is required. Run \`$SCRIPT_NAME -h\` for usage"; return 2; }
    [ "$2" ] || { _error "year is required. Run \`$SCRIPT_NAME -h\` for usage"; return 2; }
    local amount; amount="$(printf '%.2f' "$1")"
    local input_year="$2"
    local input_month="${3:-1}" # default to January

    # Validate month range before network request - an invalid month produces a
    # confusing downstream error (blank month name in the failure message)
    if [ "$input_month" -lt 1 ] || [ "$input_month" -gt 12 ]; then
        _error "month must be in range [1, 12] (got: $input_month). Run \`$SCRIPT_NAME -h\` for usage"
        return 2
    fi
    # Zero-pad month if necessary - printf is not good here because numbers with leading zeroes can be interpreted as octal
    [ "$input_month" -lt 10 ] && input_month="0${input_month#0}"

    # US Bureau of Labor Statistics inflation calculator
    local CALCULATOR_URL="https://data.bls.gov/cgi-bin/cpicalc.pl"

    # Month names array with a placeholder at index 0, so that 1 is January and 12 is December
    local MONTHS=(ERR Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec)

    # Get the most recent month of data available (e.g. 202406), from the second dropdown
    local recent_year_month; recent_year_month="$(curl -fsS "$CALCULATOR_URL" \
                                                  | grep 'selected="selected"' \
                                                  | tail -n 1 \
                                                  | sed -n 's/.*value="\([^"]*\)".*/\1/p')"
    local recent_year="${recent_year_month:0:4}"
    local recent_month="${recent_year_month:4:2}"

    # Format the input and recent year-month dates into a more readable format, e.g. "Jan 2024" rather than "202401"
    # "10#" is necessary to avoid numbers with a leading 0 from being interpreted as octal - alternatively ${MONTHS[${input_month#0}]}
    local input_date_formatted="${MONTHS[10#$input_month]} $input_year"
    local recent_date_formatted="${MONTHS[10#$recent_month]} $recent_year"

    # Fetch the data and extract the adjusted amount
    local result; result="$(curl -fsS "$CALCULATOR_URL?cost1=$amount&year1=$input_year$input_month&year2=$recent_year$recent_month" \
                            | sed -n '/id="answer"/{s/^.*\$//;s/<.*$//;s/,//g;p;}')"
    if [ -z "$result" ]; then
        _error "Failed to convert \$$amount from $input_date_formatted to $recent_date_formatted"
        _error "BLS CPI data is available for years 1913 through the most recent month (got: $input_year)"
        return 1
    fi

    # Print the result, with details to stderr only in case just the number is needed (i.e. if it's called from another script)
    echo "\$$amount: $input_date_formatted -> $recent_date_formatted" >&2
    echo "$result"
)

_inflate "$@"
__inflate_rc=$?
unset -f _inflate
if [ -n "${BASH_SOURCE[0]}" ] && [ "${BASH_SOURCE[0]}" != "$0" ]; then
    eval "unset __inflate_rc; return $__inflate_rc"
fi
eval "unset __inflate_rc; exit $__inflate_rc"