2 files changed, 38 insertions, 15 deletions

diff --git a/src/zisp/value.zig b/src/zisp/value.zig
index e8813eb..96351f2 100644
--- a/src/zisp/value.zig
+++ b/src/zisp/value.zig
@@ -214,6 +214,8 @@ pub const Zptr = *align(8) anyopaque;
 pub const PtrTag = enum(u3) {
     /// Pair aka cons cell aka *[2]Value
     pair,
+    /// Interned string buffer (8-bit length, then contents)
+    istr,
     /// Array of various types (see `Array` type for details)
     array,
     /// Procedure
diff --git a/src/zisp/value/istr.zig b/src/zisp/value/istr.zig
index c1c7093..f34dbd0 100644
--- a/src/zisp/value/istr.zig
+++ b/src/zisp/value/istr.zig
@@ -3,32 +3,54 @@ const std = @import("std");
 const value = @import("../value.zig");
 const gc = @import("../gc.zig");
 
-const ptr = @import("ptr.zig");
-const seq = @import("seq.zig");
-
 const Value = value.Value;
 
 // Zig API
 
-pub fn check(v: Value) ?*seq.Header {
-    if (v.getPtr(.seq)) |p| {
-        const h: *seq.Header = @ptrCast(p);
-        if (h.type == .string and h.info.string.interned) {
-            return h;
-        }
+/// Pointer to beginning of an interned string with a 1-byte length prefix.
+/// Aligned to 8 bytes like all Zisp heap pointers.
+pub const Istr = *align(8) struct {
+    pub fn buf(self: *@This()) [*]u8 {
+        return @ptrCast(self);
+    }
+
+    pub fn len(self: *@This()) u8 {
+        return buf()[0];
+    }
+
+    pub fn str(self: *@This()) []u8 {
+        return buf()[1 .. 1 + len()];
+    }
+};
+
+pub fn check(v: Value) ?Istr {
+    if (v.getPtr(.istr)) |p| {
+        return @ptrCast(p);
     }
     return null;
 }
 
-pub fn assert(v: Value) *seq.Header {
+pub fn assert(v: Value) Istr {
     return check(v) orelse {
         v.dump();
         @panic("not istr");
     };
 }
 
-pub fn intern(str: []const u8) Value {
-    return gc.internString(str);
+pub fn isValidIstr(s: []const u8) bool {
+    return s.len <= 255;
+}
+
+fn assertValidIstr(s: []const u8) void {
+    if (!isValidIstr(s)) {
+        std.debug.print("invalid istr: {s}\n", .{s});
+        @panic("invalid istr");
+    }
+}
+
+pub fn intern(s: []const u8) !Value {
+    assertValidIstr(s);
+    return gc.internString(s);
 }
 
 // Zisp API
@@ -38,7 +60,6 @@ pub fn pred(v: Value) Value {
 }
 
 pub fn len(v: Value) Value {
-    const l = assert(v).size;
-    std.debug.assert(l <= value.fixnum.max);
-    return value.fixnum.pack(@intCast(l));
+    const istr = assert(v);
+    return value.fixnum.pack(@intCast(istr.len()));
 }