1 files changed, 105 insertions, 0 deletions
diff --git a/src/libzisp/read.zig b/src/libzisp/read.zig
new file mode 100644
index 0000000..9ef9891
--- /dev/null
+++ b/src/libzisp/read.zig
@@ -0,0 +1,105 @@
+const std = @import("std");
+
+const gc = @import("gc.zig");
+const value = @import("value.zig");
+
+const Value = value.Value;
+
+const State = struct {
+    alloc: std.mem.Allocator,
+    input: []const u8,
+    pos: usize = 0,
+
+    next: enum {
+        start,
+
+        list,
+        list_end,
+
+        err,
+
+        done,
+    } = .start,
+
+    retval: Value = value.eof.eof,
+
+    parent: ?*State = null,
+};
+
+pub fn read(input: []const u8) Value {
+    var gpa: std.heap.GeneralPurposeAllocator(.{}) = .init;
+    var top = State{ .alloc = gpa.allocator(), .input = input };
+    var s = &top;
+    while (s.pos <= s.input.len) : (s.pos += 1) {
+        s = switch (s.next) {
+            .start => start(s),
+
+            .list => list(s),
+            .list_end => list(s),
+
+            .err => err(s),
+
+            .done => ret: {
+                if (s.parent) |parent| {
+                    s.alloc.destroy(s);
+                    break :ret parent;
+                } else {
+                    return s.retval;
+                }
+            },
+        };
+    }
+    unreachable;
+}
+
+fn start(s: *State) *State {
+    switch (s.input[s.pos]) {
+        0...8 => s.next = .err,
+
+        '\t', '\n' => {},
+
+        11...31 => s.next = .err,
+
+        ' ' => {},
+
+        '!' => s.next = .err,
+
+        '"' => quotedString(s),
+
+        else => s.next = .err,
+    }
+    return s;
+}
+
+fn quotedString(s: *State) void {
+    var buf: [6]u8 = .{0} ** 6;
+    const len = readString(&buf, s);
+    s.retval = value.pair.cons(
+        value.sstr.pack("quote"),
+        value.pair.cons(
+            value.sstr.pack(buf[0..len]),
+            value.nil.nil,
+        ),
+    );
+    s.next = .done;
+}
+
+fn readString(buf: []u8, s: *State) usize {
+    s.pos += 1; // skip opening quote
+    for (s.input[s.pos..], 0..) |c, i| {
+        if (c == '"') {
+            s.pos += i;
+            return i;
+        }
+        buf[i] = c;
+    }
+    unreachable;
+}
+
+fn list(s: *State) *State {
+    return s;
+}
+
+fn err(s: *State) *State {
+    return s;
+}