module detour;

import std.stdio;
import core.sys.windows.windows;
import core.memory;

void main() {
    hook(&testHook, &testTarget);
    testTarget();
}

void testHook() {
    writeln("hook");
}

void testTarget() {
    writeln("target");
}

HookHandle hook(void *hook, void *target) {
    auto returnAddr = new void *;
    ubyte *head = cast(ubyte *)GC.malloc(7);
    head [0..7] = cast(ubyte[])target[0 .. 7];

    // Write the trampoline code.
    ubyte *trampoline = cast(ubyte *)VirtualAlloc(null, 1024, MEM_COMMIT, PAGE_EXECUTE_READWRITE);

    /* Temporarily unhook the target function. */

    // mov eax, head
    trampoline[0 .. 3] = cast(ubyte[])[0x67, 0x66, 0xA1];
    *cast(uint *)(cast(ubyte *)trampoline + 3) = cast(uint)head;

    // mov target, eax
    trampoline[7 .. 10] = cast(ubyte[])[0x67, 0x66, 0xA3];
    *cast(uint *)(cast(ubyte *)trampoline + 10) = cast(uint)target;

    // inc eax
    trampoline[14 .. 16] = cast(ubyte[])[0x66, 0x40];

    // mov target + 1, eax
    trampoline[16 .. 19] = cast(ubyte[])[0x67, 0x66, 0xA3];
    *cast(uint *)(cast(ubyte *)trampoline + 19) = cast(uint)target;

    // inc eax
    trampoline[23 .. 25] = cast(ubyte[])[0x66, 0x40];

    // mov target + 2, eax
    trampoline[25 .. 28] = cast(ubyte[])[0x67, 0x66, 0xA3];
    *cast(uint *)(cast(ubyte *)trampoline + 28) = cast(uint)target;

    // inc eax
    trampoline[32 .. 34] = cast(ubyte[])[0x66, 0x40];

    // mov target + 3, eax
    trampoline[34 .. 37] = cast(ubyte[])[0x67, 0x66, 0xA3];
    *cast(uint *)(cast(ubyte *)trampoline + 37) = cast(uint)target;

    // inc eax
    trampoline[41 .. 43] = cast(ubyte[])[0x66, 0x40];

    // mov target + 4, eax
    trampoline[43 .. 46] = cast(ubyte[])[0x67, 0x66, 0xA3];
    *cast(uint *)(cast(ubyte *)trampoline + 46) = cast(uint)target;

    // inc eax
    trampoline[50 .. 52] = cast(ubyte[])[0x66, 0x40];

    // mov target + 5, eax
    trampoline[52 .. 55] = cast(ubyte[])[0x67, 0x66, 0xA3];
    *cast(uint *)(cast(ubyte *)trampoline + 55) = cast(uint)target;

    // inc eax
    trampoline[59 .. 61] = cast(ubyte[])[0x66, 0x40];

    // mov target + 6, eax
    trampoline[61 .. 64] = cast(ubyte[])[0x67, 0x66, 0xA3];
    *cast(uint *)(cast(ubyte *)trampoline + 64) = cast(uint)target;

    // inc eax
    trampoline[68 .. 70] = cast(ubyte[])[0x66, 0x40];

    /* Prepare to call. */

    // mov eax, [esp + 8]
    trampoline[70 .. 76] = cast(ubyte[])[0x67, 0x66, 0x8B, 0x44, 0x24, 0x08];

    // mov returnAddr, eax
    trampoline[76 .. 79] = cast(ubyte[])[0x67, 0x66, 0xA3];
    *cast(uint *)(cast(ubyte *)trampoline + 79) = cast(uint)returnAddr;

    // add esp, 4
    target[83 .. 87] = cast(ubyte[])[0x66, 0x83, 0xC4, 0x04];

    /* Call the hook. */

    // call hook
    target[87 .. 90] = cast(ubyte[])[0x67, 0xFF, 0x1D];
    *cast(uint *)(cast(ubyte *)trampoline + 90) = cast(uint)hook;

    /* Rehook the target function. */

    /* End */

    // Hook the target function.
    target[0 .. 3] = cast(ubyte[])[0x67, 0xFF, 0x2D];
    *cast(uint *)(cast(ubyte *)target + 3) = trampoline - cast(ubyte *)target;

    // Save to handle.
    return HookHandle(trampoline, head, returnAddr);
}

void unhook() {
}

public struct HookHandle {
    public ubyte *trampoline;
    public ubyte *head;
    public void **returnAddr;
}