board/slider-frame.c: Add framing for touch slider PCB

board/meson.build

@@ -9,5 +9,7 @@ board_lib = static_library(
     sources : [
         'io3.c',
         'io3.h',
+        'slider-frame.c',
+        'slider-frame.h',
     ],
 )

board/slider-frame.c

@@ -0,0 +1,179 @@
+#include <windows.h>
+
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+
+#include "board/slider-frame.h"
+
+#include "hook/iobuf.h"
+
+static void slider_frame_sync(struct iobuf *src);
+static HRESULT slider_frame_accept(const struct iobuf *dest);
+static HRESULT slider_frame_encode_byte(struct iobuf *dest, uint8_t byte);
+
+/* Frame structure:
+
+   [0] Sync byte (0xFF)
+   [1] Command
+   [2] Payload size
+   ... Payload
+   [n] Checksum: Negate the sum of all prior bytes (including sync byte!)
+
+   Byte stuffing:
+
+   0xFD is an escape byte. Un-escape the subsequent byte by adding 1. */
+
+static void slider_frame_sync(struct iobuf *src)
+{
+    size_t i;
+
+    for (i = 0 ; i < src->pos && src->bytes[i] != 0xFF ; i++);
+
+    src->pos -= i;
+    memmove(&src->bytes[0], &src->bytes[i], i);
+}
+
+static HRESULT slider_frame_accept(const struct iobuf *dest)
+{
+    uint8_t checksum;
+    size_t i;
+
+    if (dest->pos < 2 || dest->pos != dest->bytes[2] + 4) {
+        return S_FALSE;
+    }
+
+    checksum = 0;
+
+    for (i = 0 ; i < dest->pos ; i++) {
+        checksum += dest->bytes[i];
+    }
+
+    if (checksum != 0) {
+        return HRESULT_FROM_WIN32(ERROR_CRC);
+    }
+
+    return S_OK;
+}
+
+HRESULT slider_frame_decode(struct iobuf *dest, struct iobuf *src)
+{
+    uint8_t byte;
+    bool escape;
+    size_t i;
+    HRESULT hr;
+
+    assert(dest != NULL);
+    assert(dest->bytes != NULL || dest->nbytes == 0);
+    assert(dest->pos <= dest->nbytes);
+    assert(src != NULL);
+    assert(src->bytes != NULL || src->nbytes == 0);
+    assert(src->pos <= src->nbytes);
+
+    slider_frame_sync(src);
+
+    dest->pos = 0;
+    escape = false;
+
+    for (i = 0, hr = S_FALSE ; i < src->pos && hr == S_FALSE ; i++) {
+        /* Step the FSM to unstuff another byte */
+
+        byte = src->bytes[i];
+
+        if (dest->pos >= dest->nbytes) {
+            hr = HRESULT_FROM_WIN32(ERROR_INSUFFICIENT_BUFFER);
+        } else if (i == 0) {
+            dest->bytes[dest->pos++] = byte;
+        } else if (byte == 0xFF) {
+            hr = E_FAIL;
+        } else if (byte == 0xFD) {
+            if (escape) {
+                hr = E_FAIL;
+            }
+
+            escape = true;
+        } else if (escape) {
+            dest->bytes[dest->pos++] = byte + 1;
+            escape = false;
+        } else {
+            dest->bytes[dest->pos++] = byte;
+        }
+
+        /* Try to accept the packet we've built up so far */
+
+        if (SUCCEEDED(hr)) {
+            hr = slider_frame_accept(dest);
+        }
+    }
+
+    /* Handle FSM terminal state */
+
+    if (hr != S_FALSE) {
+        /* Frame was either accepted or rejected, remove it from src */
+        memmove(&src->bytes[0], &src->bytes[i], src->pos - i);
+        src->pos -= i;
+    }
+
+    return hr;
+}
+
+HRESULT slider_frame_encode(
+        struct iobuf *dest,
+        const void *ptr,
+        size_t nbytes)
+{
+    const uint8_t *src;
+    uint8_t checksum;
+    uint8_t byte;
+    size_t i;
+    HRESULT hr;
+
+    assert(dest != NULL);
+    assert(dest->bytes != NULL || dest->nbytes == 0);
+    assert(dest->pos <= dest->nbytes);
+    assert(ptr != NULL);
+
+    src = ptr;
+
+    assert(nbytes >= 2 && src[0] == 0xFF && src[2] + 3 == nbytes);
+
+    if (dest->pos >= dest->nbytes) {
+        return HRESULT_FROM_WIN32(ERROR_INSUFFICIENT_BUFFER);
+    }
+
+    dest->bytes[dest->pos++] = 0xFF;
+    checksum = 0xFF;
+
+    for (i = 1 ; i < nbytes ; i++) {
+        byte = src[i];
+        checksum += byte;
+
+        hr = slider_frame_encode_byte(dest, byte);
+
+        if (FAILED(hr)) {
+            return hr;
+        }
+    }
+
+    return slider_frame_encode_byte(dest, -checksum);
+}
+
+static HRESULT slider_frame_encode_byte(struct iobuf *dest, uint8_t byte)
+{
+    if (byte == 0xFF || byte == 0xFD) {
+        if (dest->pos + 2 > dest->nbytes) {
+            return HRESULT_FROM_WIN32(ERROR_INSUFFICIENT_BUFFER);
+        }
+
+        dest->bytes[dest->pos++] = 0xFD;
+        dest->bytes[dest->pos++] = byte - 1;
+    } else {
+        if (dest->pos + 1 > dest->nbytes) {
+            return HRESULT_FROM_WIN32(ERROR_INSUFFICIENT_BUFFER);
+        }
+
+        dest->bytes[dest->pos++] = byte;
+    }
+
+    return S_OK;
+}

board/slider-frame.h

@@ -0,0 +1,25 @@
+#pragma once
+
+#include <windows.h>
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include "hook/iobuf.h"
+
+enum {
+    SLIDER_FRAME_SYNC = 0xFF,
+};
+
+struct slider_hdr {
+    uint8_t sync;
+    uint8_t cmd;
+    uint8_t nbytes;
+};
+
+HRESULT slider_frame_decode(struct iobuf *dest, struct iobuf *src);
+
+HRESULT slider_frame_encode(
+        struct iobuf *dest,
+        const void *ptr,
+        size_t nbytes);