#!/bin/bash

set -e

if [ $# = 0 ] ; then
    exit 0
fi

from=${1:0:3}
to=${1:3:3}

get_value() {
    curl -s "http://www.xe.com/currencyconverter/convert/?Amount=1&From=$from&To=$to" | \
        grep -o "<span class='uccResultAmount'>[^<]\+" | \
        grep -o '[0-9\.]\+'
}

get_value_at_date() {
    date=$(date -d "$1" +%F)
    curl -s "http://www.xe.com/currencytables/?from=$from&date=$date" | \
        xmllint --nonet --html --xpath "//table[@id='historicalRateTbl']/tbody/tr[td/a/text() = '${to^^}']/td[3]/text()" - 2>/dev/null
}

if [ -z "$2" ] ; then
    val=$(get_value)
else
    val=$(get_value_at_date "$2")
fi

printf '%.4f\n' $val