json diff with jq

diff.jq

# Usage:
# jq -n -L . 'include "diff"; diff({hello:1}; {hello:2, world: 3})'
# {
#   "hello": {
#     "a": 1,
#     "b": 2
#   },
#   "world": {
#     "b": 3
#   }
# }
#
# produce a/b pairs for diffing values
def diff($a; $b):
  ( ( $a | type) as $at
  | ( $b | type) as $bt
  | if $at != $bt then {a: $a, b: $b}
    elif ($at == "array" or $at == "object") then
      ( [ ((($a | keys) + ($b | keys)) | unique)[] as $k
        | {
          ($k | tostring):
            ( [($a | has($k)), ($b | has($k))]
            | if . == [true, true] then diff($a[$k]; $b[$k])
              elif . == [true, false] then {a: $a[$k]}
              elif . == [false, true] then {b: $b[$k]}
              else empty # TODO: can't happen? error?
              end
            )
          }
        ]
      | add
      | if . == null then empty end
      )
    else
      if $a == $b then empty else {a: $a, b: $b} end
    end
  );