# Ruby implementation of glibc strverscmp-workalike, with some improvements.
#
# Copyright © 2021 Nick Bowler
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <https://www.gnu.org/licenses/>.
#
# This implementation is partially adapted from the GNU C Library, covered by
# the following copyright and permission notice:
#
#   Copyright (C) 1997-2018 Free Software Foundation, Inc.
#
#   The GNU C Library is free software; you can redistribute it and/or
#   modify it under the terms of the GNU Lesser General Public
#   License as published by the Free Software Foundation; either
#   version 2.1 of the License, or (at your option) any later version.

module SVC
    ZERO = "0".ord
    NINE = "9".ord
    A_UP = "A".ord
    A_LO = "a".ord
    Z_UP = "Z".ord
    Z_LO = "z".ord
    DOT  = ".".ord

    def SVC.my_ord(s)
        s ? s.ord : 0
    end

    def SVC.isdigit(x)
        return x >= ZERO && x <= NINE
    end

    def SVC.isdigitnz(x)
        return x > ZERO && x <= NINE
    end

    def SVC.isalpha(x)
        return x >= A_UP && x <= A_LO ||
               x >= A_LO && x <= Z_LO
    end

    def SVC.rank(x, xs, i)
        return 3 if isdigit(x)
        return 2 if x == DOT and isdigit(my_ord(xs[i+1]))
        return 1 if isalpha(x)
        return 0
    end
end

def strverscmp(as, bs)
    state = :normal
    i = 0

    # locate first difference, tracking context
    while true
        a, b = SVC.my_ord(as[i]), SVC.my_ord(bs[i])

        break if a == 0 or a != b
        i = i+1

        if a == SVC::ZERO
            state = :zeroes if state == :normal or state == :dot
        elsif a > SVC::ZERO and a <= SVC::NINE
            state = :integral if state == :normal or state == :dot
            state = :fractional if state == :zeroes
        elsif a == SVC::DOT
            state = :dot unless state == :normal
        else
            state = :normal
        end
    end

    # found first differing character
    diff = a - b

    case state
    when :fractional
        return diff
    when :normal
        return diff unless SVC.isdigitnz(a) and SVC.isdigitnz(b)
    when :zeroes
        ra, rb = SVC.rank(a, as, i), SVC.rank(b, bs, i)

        if ( cmp = ra <=> rb ) != 0
            return cmp if a == SVC::DOT and SVC.isalpha(b)
            return cmp if b == SVC::DOT and SVC.isalpha(a)
            return -cmp
        end

        return diff
    when :integral
        ra, rb = SVC.rank(a, as, i), SVC.rank(b, bs, i)
        return ra <=> rb if ra != rb
        return diff unless SVC.isdigit(b)
    end

    while SVC.isdigit(a)
        return 1 unless SVC.isdigit(b)
        i = i + 1
        a, b = SVC.my_ord(as[i]), SVC.my_ord(bs[i])
    end

    return -1 if SVC.isdigit(b)
    return diff
end