#! /usr/bin/env -S jq --from-file --sort-keys --compact-output
#
# Models a multiset as a JSON object.
#
# Takes an operation which contains actions, each in one of two shapes:
#
# Add x:
#
#   { "+": $x }
#
# Remove x:
#
#   { "-": $x }
#
# (where `$x` is a string).
#
# These actions are reduced to a multiset represented by an
# object. The key being the item to count (corresponding to `$x` above),
# and the value being the count itself (corresponding to how many times
# `$x` was added, minus how many times `$x` was removed).
#
# Errors if any unrecognizable action is encountered.
#
# For an example, see `rad-cob-multiset.md`.
reduce
    .op.actions[]
as {"+": $p, "-": $m} (
    .value;
    .[$p // $m // ("invalid" | halt_error)] |= (
        [. + (if $p then 1 else -1 end), 0] | max
    )
)