joeyagi/agiPatch.c

// Changes to particular games, as PLAN.md's decisions have them: copy
// protection is skipped (decision 3) and the script bugs Sierra shipped are
// patched (decision 2). A game is known by its v3 file prefix (agiRes.c).

#include "agi.h"

#include <string.h>

#include "joey/core.h"


// Gold Rush's clock: logic 0 counts v166 cycles to a second. At a change
// of speed it copies v166 from v167 (Fastest) or v165 (Fast), which the
// speed test in logic 129 sets: it counts a second of empty cycles at
// Fastest and takes a third and a quarter of them (at most 20) for the
// rate a playing cycle would run at on a 1988 PC. Here Fastest is held to
// the frame rate, playing or not, so the clock would run six times too
// fast; and the cracked copy (games/gr) lost the test with logic 129, so
// its clock stops. Both are filled with the real rates every cycle: Fast
// is one cycle a unit, Fastest one a frame (agiIdle waits for each).
#define GR_FAST_CPS_VAR     165u
#define GR_FASTEST_CPS_VAR  167u
#define GR_FAST_CPS         (AGI_MS_PER_SECOND / AGI_CYCLE_UNIT_MS)

// King's Quest IV's graveyards (rooms 16 and 18): three zombies rise at
// night, each one's distance from Rosella measured into v221..v223 and its
// state kept in v155, v156, and v162 (room 16) or v158 (room 18).
#define KQ4_GRAVEYARD_A     16u
#define KQ4_GRAVEYARD_B     18u
#define KQ4_ZOMBIE_DIST     221u
#define KQ4_ZOMBIES         3u
#define KQ4_ZOMBIE_CHASING  2u


// The copy protection (decision 3): the rooms in which a game asks a
// question from its manual, and the room a right answer goes on to.
typedef struct {
    AgiGameIdE  game;
    uint8_t     first;
    uint8_t     last;
    uint8_t     after;
} QuestionRoomsT;

// Where skipping the room would skip what a right answer does, the logic
// is changed as it loads instead: a few bytes of its code, made only over
// the bytes expected, so other releases are left alone.
#define LOGIC_PATCH_BYTES   3u

typedef struct {
    AgiGameIdE  game;
    uint8_t     logic;
    uint16_t    offset;
    uint8_t     expect[LOGIC_PATCH_BYTES];
    uint8_t     replace[LOGIC_PATCH_BYTES];
} LogicPatchT;


// ----- Tables -----

static const uint8_t kKq4ZombieStateA[KQ4_ZOMBIES] = { 155u, 156u, 162u };
static const uint8_t kKq4ZombieStateB[KQ4_ZOMBIES] = { 155u, 156u, 158u };

static const QuestionRoomsT kQuestionRooms[] = {
    // King's Quest IV asks a word from its manual in one of rooms 141 to
    // 143; the title, room 96, follows.
    { AGI_GAME_KQ4,       141u, 143u, 96u },
    // Gold Rush's claim jumper asks about the book "California Gold" on
    // the way from the title (logic 129 to room 125, which picks one of
    // rooms 126 to 128); room 73 follows. The cracked copy's logic 129 is
    // the original's logic 73 and never comes here.
    { AGI_GAME_GOLD_RUSH, 125u, 125u, 73u }
};

static const LogicPatchT kLogicPatches[] = {
    // Manhunter 1 asks about its Field Guide in room 159; a wrong answer
    // quits. random(6,61,96), which picks the question (code offset 245),
    // becomes goto 443, the right answer's path (player control, logic 90's
    // keys, room 153), as the cracked copy has it.
    { AGI_GAME_MANHUNTER_1, 159u, 245u, { 0x82u, 0x06u, 0x3Du }, { 0xFEu, 0xC3u, 0x00u } }
};


// ----- Public API (alphabetical) -----

// Before each cycle's logic 0.
void agiPatchCycle(void) {
    if (gAgi->game.id != AGI_GAME_GOLD_RUSH) {
        return;
    }
    gAgi->vars[GR_FAST_CPS_VAR]    = (uint8_t)GR_FAST_CPS;
    gAgi->vars[GR_FASTEST_CPS_VAR] = (uint8_t)jlFrameHz();
}


// The distance the distance command reports into var. King's Quest IV's
// zombie bug (in Sierra's interpreter too): with the obsidian scarab, a
// zombie rising beside Rosella is turned away before it is up, so it
// rises and never chases. Patched as ScummVM does: until a zombie chases,
// it reads as far away.
uint8_t agiPatchDistance(uint8_t var, uint8_t distance) {
    uint8_t  room;
    uint8_t  zombie;

    room = gAgi->vars[VAR_ROOM];
    if (gAgi->game.id != AGI_GAME_KQ4 || (room != KQ4_GRAVEYARD_A && room != KQ4_GRAVEYARD_B) || var < KQ4_ZOMBIE_DIST || var >= KQ4_ZOMBIE_DIST + KQ4_ZOMBIES) {
        return distance;
    }
    zombie = (uint8_t)(var - KQ4_ZOMBIE_DIST);
    if (gAgi->vars[(room == KQ4_GRAVEYARD_A) ? kKq4ZombieStateA[zombie] : kKq4ZombieStateB[zombie]] != KQ4_ZOMBIE_CHASING) {
        return AGI_DISTANCE_FAR;
    }
    return distance;
}


// A logic's code as it loads (after the header), patched in place.
void agiPatchLogic(uint8_t logicId, uint8_t *code, uint16_t length) {
    const LogicPatchT  *p;
    uint8_t             i;

    for (i = 0u; i < sizeof(kLogicPatches) / sizeof(kLogicPatches[0]); i++) {
        p = &kLogicPatches[i];
        if (gAgi->game.id == p->game && logicId == p->logic && (uint32_t)p->offset + LOGIC_PATCH_BYTES <= length && memcmp(&code[p->offset], p->expect, LOGIC_PATCH_BYTES) == 0) {
            memcpy(&code[p->offset], p->replace, LOGIC_PATCH_BYTES);
        }
    }
}


// The room new.room really goes to: a manual question is skipped.
uint8_t agiPatchRoom(uint8_t room) {
    const QuestionRoomsT  *q;
    uint8_t                i;

    for (i = 0u; i < sizeof(kQuestionRooms) / sizeof(kQuestionRooms[0]); i++) {
        q = &kQuestionRooms[i];
        if (gAgi->game.id == q->game && room >= q->first && room <= q->last) {
            return q->after;
        }
    }
    return room;
}