stop vendoring the sqlite C code, rework the build

* Use the zig package manager to fetch sqlite directly from upstream
* Integrate the preprocessing tool directly into the build script

This makes it simpler to upgrade the SQLite source code:
* use `zig fetch`
* run `zig build preprocess-headers`

1 files changed, 236 insertions, 0 deletions

diff --git a/build/Preprocessor.zig b/build/Preprocessor.zig
new file mode 100644
index 0000000..a069523
--- /dev/null
+++ b/build/Preprocessor.zig
@@ -0,0 +1,236 @@
+const std = @import("std");
+const debug = std.debug;
+const mem = std.mem;
+
+// This tool is used to preprocess the sqlite3 headers to make them usable to build loadable extensions.
+//
+// Due to limitations of `zig translate-c` (used by @cImport) the code produced by @cImport'ing the sqlite3ext.h header is unusable.
+// The sqlite3ext.h header redefines the SQLite API like this:
+//
+//     #define sqlite3_open_v2 sqlite3_api->open_v2
+//
+// This is not supported by `zig translate-c`, if there's already a definition for a function the aliasing macros won't do anything:
+// translate-c keeps generating the code for the function defined in sqlite3.h
+//
+// Even if there's no definition already (we could for example remove the definition manually from the sqlite3.h file),
+// the code generated fails to compile because it references the variable sqlite3_api which is not defined
+//
+// And even if the sqlite3_api is defined before, the generated code fails to compile because the functions are defined as consts and
+// can only reference comptime stuff, however sqlite3_api is a runtime variable.
+//
+// The only viable option is to completely reomve the original function definitions and redefine all functions in Zig which forward
+// calls to the sqlite3_api object.
+//
+// This works but it requires fairly extensive modifications of both sqlite3.h and sqlite3ext.h which is time consuming to do manually;
+// this tool is intended to automate all these modifications.
+
+fn readOriginalData(allocator: mem.Allocator, path: []const u8) ![]const u8 {
+    var file = try std.fs.cwd().openFile(path, .{});
+    defer file.close();
+
+    var reader = file.reader();
+
+    const data = reader.readAllAlloc(allocator, 1024 * 1024);
+    return data;
+}
+
+const Processor = struct {
+    const Range = union(enum) {
+        delete: struct {
+            start: usize,
+            end: usize,
+        },
+        replace: struct {
+            start: usize,
+            end: usize,
+            replacement: []const u8,
+        },
+    };
+
+    allocator: mem.Allocator,
+
+    data: []const u8,
+    pos: usize,
+
+    range_start: usize,
+    ranges: std.ArrayList(Range),
+
+    fn init(allocator: mem.Allocator, data: []const u8) !Processor {
+        return .{
+            .allocator = allocator,
+            .data = data,
+            .pos = 0,
+            .range_start = 0,
+            .ranges = try std.ArrayList(Range).initCapacity(allocator, 4096),
+        };
+    }
+
+    fn readable(self: *Processor) []const u8 {
+        if (self.pos >= self.data.len) return "";
+
+        return self.data[self.pos..];
+    }
+
+    fn previousByte(self: *Processor) ?u8 {
+        if (self.pos <= 0) return null;
+        return self.data[self.pos - 1];
+    }
+
+    fn skipUntil(self: *Processor, needle: []const u8) bool {
+        const pos = mem.indexOfPos(u8, self.data, self.pos, needle);
+        if (pos) |p| {
+            self.pos = p;
+            return true;
+        }
+        return false;
+    }
+
+    fn consume(self: *Processor, needle: []const u8) void {
+        debug.assert(self.startsWith(needle));
+
+        self.pos += needle.len;
+    }
+
+    fn startsWith(self: *Processor, needle: []const u8) bool {
+        if (self.pos >= self.data.len) return false;
+
+        const data = self.data[self.pos..];
+        return mem.startsWith(u8, data, needle);
+    }
+
+    fn rangeStart(self: *Processor) void {
+        self.range_start = self.pos;
+    }
+
+    fn rangeDelete(self: *Processor) void {
+        self.ranges.appendAssumeCapacity(Range{
+            .delete = .{
+                .start = self.range_start,
+                .end = self.pos,
+            },
+        });
+    }
+
+    fn rangeReplace(self: *Processor, replacement: []const u8) void {
+        self.ranges.appendAssumeCapacity(Range{
+            .replace = .{
+                .start = self.range_start,
+                .end = self.pos,
+                .replacement = replacement,
+            },
+        });
+    }
+
+    fn dump(self: *Processor, writer: anytype) !void {
+        var pos: usize = 0;
+        for (self.ranges.items) |range| {
+            switch (range) {
+                .delete => |dr| {
+                    const to_write = self.data[pos..dr.start];
+                    try writer.writeAll(to_write);
+                    pos = dr.end;
+                },
+                .replace => |rr| {
+                    const to_write = self.data[pos..rr.start];
+                    try writer.writeAll(to_write);
+                    try writer.writeAll(rr.replacement);
+                    pos = rr.end;
+                },
+            }
+
+            // debug.print("excluded range: start={d} end={d} slice=\"{s}\"\n", .{
+            //     range.start,
+            //     range.end,
+            //     processor.data[range.start..range.end],
+            // });
+        }
+
+        // Finally append the remaining data in the buffer (the last range will probably not be the end of the file)
+        if (pos < self.data.len) {
+            const remaining_data = self.data[pos..];
+            try writer.writeAll(remaining_data);
+        }
+    }
+};
+
+pub fn sqlite3(allocator: mem.Allocator, input_path: []const u8, output_path: []const u8) !void {
+    const data = try readOriginalData(allocator, input_path);
+
+    var processor = try Processor.init(allocator, data);
+
+    while (true) {
+        // Everything function definition is declared with SQLITE_API.
+        // Stop the loop if there's none in the remaining data.
+        if (!processor.skipUntil("SQLITE_API ")) break;
+
+        // If the byte just before is not a LN it's not a function definition.
+        // There are a couple instances where SQLITE_API appears in a comment.
+        const previous_byte = processor.previousByte() orelse 0;
+        if (previous_byte != '\n') {
+            processor.consume("SQLITE_API ");
+            continue;
+        }
+
+        // Now we assume we're at the start of a function definition.
+        //
+        // We keep track of every function definition by marking its start and end position in the data.
+
+        processor.rangeStart();
+
+        processor.consume("SQLITE_API ");
+        if (processor.startsWith("SQLITE_EXTERN ")) {
+            // This is not a function definition, ignore it.
+            // try processor.unmark();
+            continue;
+        }
+
+        _ = processor.skipUntil(");\n");
+        processor.consume(");\n");
+
+        processor.rangeDelete();
+    }
+
+    // Write the result
+
+    var output_file = try std.fs.cwd().createFile(output_path, .{ .mode = 0o0644 });
+    defer output_file.close();
+
+    try output_file.writeAll("/* sqlite3.h edited by the zig-sqlite build script */\n");
+    try processor.dump(output_file.writer());
+}
+
+pub fn sqlite3ext(allocator: mem.Allocator, input_path: []const u8, output_path: []const u8) !void {
+    const data = try readOriginalData(allocator, input_path);
+
+    var processor = try Processor.init(allocator, data);
+
+    // Replace the include line
+
+    debug.assert(processor.skipUntil("#include \"sqlite3.h\""));
+
+    processor.rangeStart();
+    processor.consume("#include \"sqlite3.h\"");
+    processor.rangeReplace("#include \"loadable-ext-sqlite3.h\"");
+
+    // Delete all #define macros
+
+    while (true) {
+        if (!processor.skipUntil("#define sqlite3_")) break;
+
+        processor.rangeStart();
+
+        processor.consume("#define sqlite3_");
+        _ = processor.skipUntil("\n");
+        processor.consume("\n");
+
+        processor.rangeDelete();
+    }
+
+    // Write the result
+
+    var output_file = try std.fs.cwd().createFile(output_path, .{ .mode = 0o0644 });
+    defer output_file.close();
+
+    try output_file.writeAll("/* sqlite3ext.h edited by the zig-sqlite build script */\n");
+    try processor.dump(output_file.writer());
+}