#!/bin/sh

set -e

# accepts two semantic-ish versions and echoes a character representing the
# relationship between the two.
# "semantic-ish" means here "a superset of semantic versioning."  all correctly
# constructed semantic versions should compare in accordance with version 2.0 of
# the semver.org spec.  versions that do not conform to the spec may also
# compare correctly.  the primary difference is that there is no limitation on
# the number of version fields.
comparever() { comparever_default $@; }

# a split definition allows other formats to use this function to define their
# own comparever functions if they wish.
comparever_default() {
    v1="$1"
    v2="$2"

    # strip any trailing meta tags
    v1="${v1%+*}"
    v2="${v2%+*}"

    # equal?
    [ "$v1" != "$v2" ] || { echo "=" && return 0; }

    # strip leading characters that match.
    while [ "$(echo "$v1" | cut -c1)" = "$(echo "$v2" | cut -c1)" ]; do
        v1="$(echo "$v1" | cut -c2-)"
        v2="$(echo "$v2" | cut -c2-)"
    done
    v1head="$(echo "$v1" | cut -c1)"
    v2head="$(echo "$v2" | cut -c1)"

    # if the difference is a prerelease tag, the prerelease is lesser.
    case "$v1head" in -) echo '<' && return 0;; esac
    case "$v2head" in -) echo '>' && return 0;; esac

    # otherwise, if one is a prefix of the other, the other is greater.
    [ -n "$v1" ] || { echo '<' && return 0; }
    [ -n "$v2" ] || { echo '>' && return 0; }

    # otherwise, if one starts with a dot, the other is the greater.
    case "$v1head" in .) echo '<' && return 0;; esac
    case "$v2head" in .) echo '>' && return 0;; esac

    # strip any remaining version fields
    v1="$(echo "$v1" | sed s/\\..*\$//)"
    v2="$(echo "$v2" | sed s/\\..*\$//)"

    # if the version tails are numeric, compare numerically.  (ignore prerelease
    # tags.) otherwise, compare lexically.
    if (echo "${v1%-*}${v2%-*}" | grep -q "^[0-9]\+\$"); then
        [ "$(echo "${v1%-*}")" -gt "$(echo "${v2%-*}")" ] \
        && echo ">" \
        || echo "<"

    else
        # v1 and v2 contain only the differences.  using 'cut' and tabs here to
        # perform a lexical sort on the differences and retrieve the original
        # string associated with the greater of the two remainders.
        greater=$(printf "%s\t%s\n%s\t%s" "$v1" "$1" "$v2" "$2" \
                 | sort | tail -n1 | cut -f2)

        [ "$greater" = "$1" ] && echo ">" || echo "<"
    fi
}