Trap trigger_weight json filed is ignored

[hexagonrecursion]

Describe the bug

While working on #36110 I noticed a strange phenomenon: even though json sets the trigger weight of all traps as 200 grams (see below), the experiments show that the smallest weight that actually triggers a trap is >500 grams.

Cataclysm-DDA/data/json/traps.json

Lines 141 to 142 in adb3d75

	"id": "tr_beartrap",
	"trigger_weight": 200,

After some code spelunking I discovered the cause:

Cataclysm-DDA/src/ballistics.cpp

Lines 133 to 143 in 43eebbd

	if( effects.count( "HEAVY_HIT" ) ) {
	if( here.has_flag( flag_LIQUID, pt ) ) {
	sounds::sound( pt, 10, sounds::sound_t::combat, _( "splash!" ), false, "bullet_hit", "hit_water" );
	} else {
	sounds::sound( pt, 8, sounds::sound_t::combat, _( "thud." ), false, "bullet_hit", "hit_wall" );
	}
	const trap &tr = here.tr_at( pt );
	if( tr.triggered_by_item( dropped_item ) ) {
	tr.trigger( pt, nullptr, &dropped_item );
	}
	}

Cataclysm-DDA/src/ranged.cpp

Lines 1015 to 1017 in c2b5b38

	if( weight > 500_gram ) {
	proj_effects.insert( "HEAVY_HIT" );
	}

Steps To Reproduce

1. Set up a bear trap
2. Throw a canvas bag with 165 papers at it ~ 0.5 kg
3. Note that it did not trigger
4. Throw a canvas bag with 166 papers at it ~ 0.5 kg
5. Now it triggers

Expected behavior

Either the game should respect the trigger_weight json field, or this filed should be removed as dead code (every trap that lists trigger_weight sets it to 200 grams).

Screenshots

none

Versions and configuration

• OS: Linux
  • OS Version: LSB Version: :core-4.1-amd64:core-4.1-noarch; Distributor ID: Fedora; Description: Fedora release 32 (Thirty Two); Release: 32; Codename: ThirtyTwo;
• Game Version: 0.E-3470-ga87254b [64-bit]
• Graphics Version: Tiles
• Game Language: System language []
• Mods loaded: [
  Dark Days Ahead [dda],
  Disable NPC Needs [no_npc_food]
  ]

Additional context

Does the code have to be this complicated? So many knobs and dials. So much indirection. This is much simpler (pseudocode)

if item.weight() > gram(500):
    trap.trigger()

[hexagonrecursion]

To clarify my last point:

1. Fact: drop_or_embed_projectile wants to check whether the thrown item is heavier than a certain threshold, but instead of doing the comparison directly it relies on a different function in a different file to have performed the comparison and have set a flag.
2. Opinion: I consider this a form of indirection. Moreover: I consider this indirection to be premature.
3. As far as I can tell the trigger_weight of a trap only affects whether a thrown item can trigger it. It has no effect on whether a monster or the player can trigger it.
4. Fact: trigger_weight of traps was jsonized
5. As far as I can tell all traps have the same trigger_weight: 200g
6. Opinion: I consider jsonization to also be a form of indirection. And also premature in this case.
7. Fact: this is a very roundabout way of implementing a simple behavior: a thrown item triggers the trap if it is heavier than 500g.
8. Opinion: This bug went unnoticed for years. I consider this to be evidence that for gameplay purposes this simple behavior is just fine.