From af98fa608fb4ba98138e2b1239f173e6d74d6d86 Mon Sep 17 00:00:00 2001
From: Bad Diode <bd@badd10de.dev>
Date: Fri, 21 May 2021 17:17:52 +0200
Subject: Add incomplete filesystem prototype

---
 src/filesystem.c | 118 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 118 insertions(+)
 create mode 100644 src/filesystem.c

(limited to 'src/filesystem.c')

diff --git a/src/filesystem.c b/src/filesystem.c
new file mode 100644
index 0000000..2da77be
--- /dev/null
+++ b/src/filesystem.c
@@ -0,0 +1,118 @@
+// We need 64 * 32 bytes (2K) of SRAM for file indexes. To avoid corruption
+// issues we ignore the first file (32 bytes).
+// Note that the filename should include the null terminator if we want to use
+// strcmp.
+#define FILE_NAME_SIZE     27
+#define FILE_CAPACITY      63
+#define FILE_HEADER_OFFSET 2
+#define FILE_INDEX_OFFSET  32
+#define FILE_DATA_OFFSET   KB(2)
+#define FILE_MAX_SIZE      KB(1)
+#define SRAM               ((vu8*)(MEM_CART))
+
+typedef struct File {
+    char name[FILE_NAME_SIZE + 1];
+    u16 size;
+    u16 mem_offset; // NOTE: Unused...
+} File;
+
+// The filesystem header.
+typedef struct FileSystem {
+    u16 num_files;
+    u16 data_size; // NOTE: Unused...
+    u16 data_capacity; // NOTE: Unused...
+    File files[FILE_CAPACITY];
+} FileSystem;
+
+static FileSystem filesystem;
+
+void
+_fs_read(u8 *dst, size_t pos, size_t n_bytes) {
+    for (size_t i = 0; i < n_bytes; ++i) {
+        dst[i] = SRAM[pos + i];
+    }
+}
+
+void
+_fs_write(u8 *src, size_t pos, size_t n_bytes) {
+    for (size_t i = 0; i < n_bytes; ++i) {
+        SRAM[pos + i] = src[i];
+    }
+}
+
+void
+fs_init() {
+    // Load header if existing.
+    _fs_read(&filesystem, FILE_HEADER_OFFSET, offsetof(FileSystem, files));
+    if (filesystem.num_files == 0xFFFF
+            && filesystem.data_capacity == 0xFFFF
+            && filesystem.data_size == 0xFFFF) {
+        filesystem.num_files = 0;
+        filesystem.data_size = 0;
+        filesystem.data_capacity = 27 * FILE_MAX_SIZE;
+        memset(&filesystem.files, 0, FILE_CAPACITY * sizeof(File));
+        _fs_write(&filesystem, FILE_HEADER_OFFSET, offsetof(FileSystem, files));
+    } else {
+        _fs_read(&filesystem.files, FILE_INDEX_OFFSET, sizeof(File) * filesystem.num_files);
+    }
+}
+
+int
+fs_open_file(char *name) {
+    // Try to find an existing file.
+    for (size_t i = 0; i < filesystem.num_files; ++i) {
+        // TODO: Replace strcmp with vectorized fixed size char comparison.
+        if (strcmp(name, filesystem.files[i].name) == 0) {
+            return i;
+        }
+    }
+    // Create a new file if there is space.
+    if (filesystem.num_files < FILE_CAPACITY) {
+        size_t index = filesystem.num_files++;
+        size_t k = 0;
+        while(*name) {
+            filesystem.files[index].name[k++] = *name++;
+        }
+        filesystem.files[index].size = 0;
+        filesystem.files[index].mem_offset = 0;
+
+        // Update file index.
+        _fs_write(&filesystem.files[index],
+                FILE_INDEX_OFFSET + index * sizeof(File),
+                sizeof(File));
+
+        // Update header.
+        _fs_write(&filesystem, FILE_HEADER_OFFSET, offsetof(FileSystem, files));
+
+        return index;
+    }
+    return -1;
+}
+
+int
+fs_write(u8 *src, size_t n_bytes, u16 file_index, u16 offset, bool append) {
+    File *file = &filesystem.files[file_index];
+
+    // Check if there is enough capacity for this write operation.
+    if (offset + n_bytes >= FILE_MAX_SIZE) {
+        return -1;
+    }
+
+    // Write data to file block.
+    _fs_write(src, FILE_DATA_OFFSET + FILE_MAX_SIZE * file_index + offset, n_bytes);
+
+    // Update file index.
+    if (append) {
+        if (offset + n_bytes > file->size) {
+            file->size = offset + n_bytes;
+        }
+    } else {
+        file->size = offset + n_bytes;
+    }
+    _fs_write(file, FILE_INDEX_OFFSET + file_index * sizeof(File), sizeof(File));
+
+    // Update header.
+    _fs_write(&filesystem, FILE_HEADER_OFFSET, offsetof(FileSystem, files));
+
+    return n_bytes;
+}
-- 
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