update

13 files changed, 421 insertions, 154 deletions

diff --git a/src/libzisp/gc.zig b/src/libzisp/gc.zig
new file mode 100644
index 0000000..819fa0b
--- /dev/null
+++ b/src/libzisp/gc.zig
@@ -0,0 +1,15 @@
+const std = @import("std");
+
+const Value = @import("value.zig").Value;
+
+var _gpa: std.heap.GeneralPurposeAllocator(.{}) = .init;
+const gpa = _gpa.allocator();
+
+pub const Bucket = packed union {
+    bits: u64,
+    value: Value,
+};
+
+pub fn alloc(count: usize) []Bucket {
+    return gpa.alloc(Bucket, count) catch @panic("OOM");
+}
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 = ⊤
+    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;
+}
diff --git a/src/libzisp/value.zig b/src/libzisp/value.zig
index da67af7..dd9df3c 100644
--- a/src/libzisp/value.zig
+++ b/src/libzisp/value.zig
@@ -58,28 +58,26 @@
 //
 // === Pointers ===
 //
-// Pointers are further subdivided as follows based on the remaining 51 bits:
+// Pointers are further subdivided as follows based on the remaining 51 bits,
+// with the first three bits used as a sort of tag:
 //
-//   MSb = 1          :: Foreign Pointer (or a "special 50-bit fixnum")
+//   000   :: Pointer to Zisp heap object (string, vector, etc.)
 //
-//   MSb = 0, SSb = 0 :: Pointer to heap object (string, vector, etc.)
+//   001   :: Weak pointer to Zisp heap object
 //
-//   MSb = 0, SSb = 1 :: Weak pointer to heap object
+//   01.   :: Undefined
 //
-//   (SSb = Second-most significant bit)
-//
-// This means regular pointers to the Zisp heap are 49 bits.  Of these, we only
-// really need 45, since 64-bit platforms are in practice limited to 48-bit
-// addresses, and allocations happen at 8-byte boundaries, meaning the least
-// significant 3 bit are always 0.  Thus, we are able to store 4-bit tags in
-// those 49-bit pointers alongside the actual, multiple-of-8, 48-bit address.
+//   1..   :: Undefined
 //
-// Note that foreign pointers avoid stepping on any forbidden value, thanks to
-// bit 51 being set.
+// This means pointers to the Zisp heap are 48 bits.  Of those, we only really
+// need 45, since 64-bit platforms are in practice limited to 48-bit addresses,
+// and allocations happen at 8-byte boundaries, meaning the least significant 3
+// bit are always 0.  Thus, we are able to store yet another 3-bit tag in those
+// 48-bit pointers alongside the actual, multiple-of-8, 48-bit address.
 //
 // The forbidden value 3, Positive cqNaN, is avoided thanks to the fact that a
 // regular Zisp heap pointer can never be null.  Weak pointers, which can be
-// null, avoid stepping on that forbidden value thanks to bit 50 being set.
+// null, avoid stepping on that forbidden value thanks to bit 49 being set.
 //
 //
 // === Other values ===
@@ -88,7 +86,7 @@
 //
 //   000   :: Undefined
 //
-//   001   :: Small string
+//   001   :: Short string
 //
 //   010   :: Unicode code point
 //
@@ -106,14 +104,16 @@
 //
 // There may also be uninterned strings on the heap that are also as short but
 // ended up on the heap due to being uninterned.  Calling intern on them will
-// return the equivalent small string.
+// return the equivalent short string.
 //
 // Unicode code points need a maximum of 21 bits, yet we have 48 available.
 // This may be exploited for a future extension.
 //
 // Similarly, it's extremely unlikely that we will ever need more than a few
 // dozen singleton values (false, true, null, and so on).  As such, this range
-// of bit patterns may be subdivided further in the future.
+// of bit patterns may be subdivided in the future.  Right now, only the lowest
+// 8 bits are allowed to be set, with the other 40 being reserved, so there's a
+// limit of 256 singleton values that can be defined.
 //
 // And on top of all that we still have a 48-bit and a 50-bit range left!
 //
@@ -142,72 +142,109 @@ pub const boole = @import("value/boole.zig");
 pub const nil = @import("value/nil.zig");
 pub const eof = @import("value/eof.zig");
 
+pub const pair = @import("value/pair.zig");
+
 /// To fill up the u11 exponent part of a NaN.
 const FILL = 0x7ff;
 
 /// Represents a Zisp value/object.
 pub const Value = packed union {
+    /// To get the value as a regular double.
     double: f64,
+
+    /// To get an agnostic value for direct comparison with == i.e. eq?.
     bits: u64,
 
-    nan: packed struct {
+    // Some of the structs below are just for inspection, whereas others are to
+    // initialize a new value of that category as well as read it that way.
+
+    /// Inspection through the lens of the general IEEE 754 double layout.
+    ieee: packed struct {
         rest: u51,
-        quiet: u1,
-        exp: u11 = FILL,
-        sign: u1,
+        quiet: bool,
+        exp: u11,
+        sign: bool,
     },
 
+    /// For initializing and reading fixnums.
     fixnum: packed struct {
         code: u51,
         negative: bool,
         _: u11 = FILL,
-        is_fixnum: bool = true,
+        _is_fixnum: bool = true,
     },
 
+    /// Inspection through the lens of the ptr category.
     ptr: packed struct {
-        // if foreign, we don't actually use value and is_weak
-        value: u49,
-        weak: bool = false,
-        foreign: bool = false,
-        is_ptr: bool = true,
-        _: u11 = FILL,
-        _fixnum: bool = false,
+        _value: u48,
+        is_weak: bool,
+        _unused: bool,
+        is_foreign: bool,
+        _is_ptr: bool,
+        _: u11,
+        _is_fixnum: bool,
     },
 
+    /// For initializing and reading foreign pointers.
     fptr: packed struct {
         value: u50,
-        _foreign: bool = true,
-        _ptr: bool = true,
+        _is_foreign: bool = true,
+        _is_ptr: bool = true,
         _: u11 = FILL,
-        _fixnum: bool = false,
+        _is_fixnum: bool = false,
+    },
+
+    /// For initializing and reading Zisp heap pointers.
+    zptr: packed struct {
+        tagged_value: u48,
+        is_weak: bool = false,
+        _unused: bool = false,
+        _is_foreign: bool = false,
+        _is_ptr: bool = true,
+        _: u11 = FILL,
+        _is_fixnum: bool = false,
+    },
+
+    /// Inspection as an other (non-fixnum, non-pointer) packed value.
+    other: packed struct {
+        _value: u48,
+        tag: OtherTag,
+        _is_ptr: bool,
+        _: u11,
+        _is_ifxnum: bool,
     },
 
+    /// For initializing and reading short strings.
     sstr: packed struct {
-        // packed struct cannot contain array
-        value: u48,
-        tag: Tag = .sstr,
-        ptr: bool = false,
+        // actually [6]u8 but packed struct cannot contain arrays
+        string: u48,
+        _tag: OtherTag = .sstr,
+        _is_ptr: bool = false,
         _: u11 = FILL,
-        fixnum: bool = false,
+        _is_fixnum: bool = false,
     },
 
+    /// For initializing and reading characters.
     char: packed struct {
-        value: u48,
-        tag: Tag = .char,
-        ptr: bool = false,
+        char: u21,
+        _reserved: u27 = 0,
+        _tag: OtherTag = .char,
+        _is_ptr: bool = false,
         _: u11 = FILL,
-        fixnum: bool = false,
+        _is_fixnum: bool = false,
     },
 
+    /// For initializing and reading misc values aka singletons.
     misc: packed struct {
-        value: u48,
-        tag: Tag = .misc,
-        ptr: bool = false,
+        value: u8,
+        _reserved: u40 = 0,
+        _tag: OtherTag = .misc,
+        _is_ptr: bool = false,
         _: u11 = FILL,
-        fixnum: bool = false,
+        _is_fixnum: bool = false,
     },
 
-    const Tag = enum(u3) { sstr = 1, char = 2, misc = 3 };
+    const OtherTag = enum(u3) { sstr = 1, char = 2, misc = 3 };
 
     const Self = @This();
 
@@ -216,8 +253,36 @@ pub const Value = packed union {
         std.debug.dumpHex(std.mem.asBytes(&self));
     }
 
-    /// Checks for a Zisp value (non-double) packed into a NaN.
+    // The following aren't type predicates per se, but rather determine which
+    // general category the value is in.  The exceptions are fixnum and double,
+    // since those aren't sub-categorized into further types.
+
+    /// Checks for a Zisp double, including: +nan.0, -nan.0, +inf.0, -inf.0
+    pub fn isDouble(self: Self) bool {
+        return self.ieee.exp != FILL or self.ieee.rest == 0;
+    }
+
+    /// Checks for a non-double Zisp value packed into a NaN.
     pub fn isPacked(self: Self) bool {
-        return self.nan.exp == FILL and self.nan.rest != 0;
+        return !self.isDouble();
+    }
+
+    /// Checks for a fixnum.
+    pub fn isFixnum(self: Self) bool {
+        return self.isPacked() and self.ieee.sign;
+    }
+
+    /// Checks for any kind of pointer.
+    pub fn isPtr(self: Self) bool {
+        return self.isPacked() and !self.ieee.sign and self.ieee.quiet;
+    }
+
+    /// Checks for a non-double, non-fixnum, non-pointer Zisp value.
+    fn _isOther(self: Self) bool {
+        return self.isPacked() and !self.ieee.sign and !self.ieee.quiet;
+    }
+
+    pub fn isOther(self: Self, tag: OtherTag) bool {
+        return self._isOther() and self.other.tag == tag;
     }
 };
diff --git a/src/libzisp/value/boole.zig b/src/libzisp/value/boole.zig
index 0af7e22..623dbc2 100644
--- a/src/libzisp/value/boole.zig
+++ b/src/libzisp/value/boole.zig
@@ -30,5 +30,5 @@ pub fn unpack(v: Value) bool {
 // Zisp API
 
 pub fn pred(v: Value) Value {
-    return if (check(v)) t else f;
+    return pack(check(v));
 }
diff --git a/src/libzisp/value/char.zig b/src/libzisp/value/char.zig
index 6a38f0d..98bb26f 100644
--- a/src/libzisp/value/char.zig
+++ b/src/libzisp/value/char.zig
@@ -1,10 +1,11 @@
-const Value = @import("../value.zig").Value;
+const value = @import("../value.zig");
+
+const Value = value.Value;
+
+// Zig API
 
 pub fn check(v: Value) bool {
-    return v.isPacked() and
-        !v.char.fixnum and
-        !v.char.ptr and
-        v.char.tag == .char;
+    return v.isOther(.char);
 }
 
 pub fn assert(v: Value) void {
@@ -15,10 +16,16 @@ pub fn assert(v: Value) void {
 }
 
 pub fn pack(c: u21) Value {
-    return .{ .char = .{ .value = c } };
+    return .{ .char = .{ .char = c } };
 }
 
 pub fn unpack(v: Value) u21 {
     assert(v);
-    return @truncate(v.char.value);
+    return @truncate(v.char.char);
+}
+
+// Zisp API
+
+pub fn pred(v: Value) Value {
+    return value.boole.pack(check(v));
 }
diff --git a/src/libzisp/value/double.zig b/src/libzisp/value/double.zig
index 5c98324..5cfe6ee 100644
--- a/src/libzisp/value/double.zig
+++ b/src/libzisp/value/double.zig
@@ -1,10 +1,11 @@
-const Value = @import("../value.zig").Value;
+const value = @import("../value.zig");
+const Value = value.Value;
 
 // Zig API
 
 /// Checks for a Zisp double (double, +inf, -inf, or canonical NaN).
 pub fn check(v: Value) bool {
-    return !v.isPacked();
+    return v.isDouble();
 }
 
 /// Asserts check().
@@ -27,7 +28,7 @@ pub fn unpack(v: Value) f64 {
 // Zisp API
 
 pub fn pred(v: Value) Value {
-    return Value.boole.pack(check(v));
+    return value.boole.pack(check(v));
 }
 
 pub fn add(v1: Value, v2: Value) Value {
diff --git a/src/libzisp/value/eof.zig b/src/libzisp/value/eof.zig
index 34ab35d..367a86c 100644
--- a/src/libzisp/value/eof.zig
+++ b/src/libzisp/value/eof.zig
@@ -1,7 +1,8 @@
-const Value = @import("../value.zig").Value;
-const misc = @import("misc.zig");
+const value = @import("../value.zig");
 
-pub const eof = misc.eof;
+const Value = value.Value;
+
+pub const eof = @import("misc.zig").eof;
 
 // Zig API
 
@@ -23,5 +24,5 @@ pub fn get() Value {
 }
 
 pub fn pred(v: Value) Value {
-    return if (check(v)) misc.t else misc.f;
+    return value.boole.pack(check(v));
 }
diff --git a/src/libzisp/value/fixnum.zig b/src/libzisp/value/fixnum.zig
index 6d26a9c..888dd3a 100644
--- a/src/libzisp/value/fixnum.zig
+++ b/src/libzisp/value/fixnum.zig
@@ -1,12 +1,13 @@
 const std = @import("std");
+const value = @import("../value.zig");
 
-const Value = @import("../value.zig").Value;
+const Value = value.Value;
 
 // Zig API
 
 /// Checks for a Zisp fixnum.
 pub fn check(v: Value) bool {
-    return v.isPacked() and v.fixnum.is_fixnum;
+    return v.isFixnum();
 }
 
 /// Asserts check().
@@ -77,7 +78,7 @@ pub fn unpack(v: Value) i64 {
 // Zisp API
 
 pub fn pred(v: Value) Value {
-    return Value.boole.pack(check(v));
+    return value.boole.pack(check(v));
 }
 
 pub fn add(v1: Value, v2: Value) Value {
diff --git a/src/libzisp/value/misc.zig b/src/libzisp/value/misc.zig
index 793c60e..30cbf84 100644
--- a/src/libzisp/value/misc.zig
+++ b/src/libzisp/value/misc.zig
@@ -4,3 +4,5 @@ pub const f = Value{ .misc = .{ .value = 0 } };
 pub const t = Value{ .misc = .{ .value = 1 } };
 pub const nil = Value{ .misc = .{ .value = 2 } };
 pub const eof = Value{ .misc = .{ .value = 3 } };
+
+pub const undef = Value{ .misc = .{ .value = 255 } };
diff --git a/src/libzisp/value/nil.zig b/src/libzisp/value/nil.zig
index 1b1a51e..14bd800 100644
--- a/src/libzisp/value/nil.zig
+++ b/src/libzisp/value/nil.zig
@@ -1,7 +1,8 @@
-const Value = @import("../value.zig").Value;
-const misc = @import("misc.zig");
+const value = @import("../value.zig");
 
-pub const nil = misc.nil;
+const Value = value.Value;
+
+pub const nil = @import("misc.zig").nil;
 
 // Zig API
 
@@ -23,5 +24,5 @@ pub fn get() Value {
 }
 
 pub fn pred(v: Value) Value {
-    return if (check(v)) misc.t else misc.f;
+    return value.boole.pack(check(v));
 }
diff --git a/src/libzisp/value/pair.zig b/src/libzisp/value/pair.zig
new file mode 100644
index 0000000..541a5f5
--- /dev/null
+++ b/src/libzisp/value/pair.zig
@@ -0,0 +1,57 @@
+const std = @import("std");
+const value = @import("../value.zig");
+const gc = @import("../gc.zig");
+
+const ptr = @import("ptr.zig");
+
+const Value = value.Value;
+
+// Zig API
+
+pub fn check(v: Value) bool {
+    return ptr.checkZisp(v, .pair);
+}
+
+pub fn assert(v: Value) void {
+    if (!check(v)) {
+        v.dump();
+        @panic("not pair");
+    }
+}
+
+// Zisp API
+
+pub fn pred(v: Value) Value {
+    return value.boole.pack(check(v));
+}
+
+pub fn cons(v1: Value, v2: Value) Value {
+    const mem = gc.alloc(2);
+    mem[0] = .{ .value = v1 };
+    mem[1] = .{ .value = v2 };
+    return ptr.pack(mem.ptr, .pair);
+}
+
+fn getMem(v: Value) *[2]Value {
+    return @ptrCast(ptr.unpack(v).@"0");
+}
+
+pub fn car(v: Value) Value {
+    assert(v);
+    return getMem(v)[0];
+}
+
+pub fn cdr(v: Value) Value {
+    assert(v);
+    return getMem(v)[1];
+}
+
+pub fn setcar(v: Value, new: Value) void {
+    assert(v);
+    getMem(v)[0] = new;
+}
+
+pub fn setcdr(v: Value, new: Value) void {
+    assert(v);
+    getMem(v)[1] = new;
+}
diff --git a/src/libzisp/value/ptr.zig b/src/libzisp/value/ptr.zig
index 6a3a6c4..fe13af5 100644
--- a/src/libzisp/value/ptr.zig
+++ b/src/libzisp/value/ptr.zig
@@ -1,11 +1,14 @@
 const std = @import("std");
+const value = @import("../value.zig");
+const gc = @import("../gc.zig");
 
-const Value = @import("../value.zig").Value;
+const Bucket = gc.Bucket;
+const Value = value.Value;
 
 // Zig API
 
 pub fn check(v: Value) bool {
-    return v.isPacked() and v.ptr.is_ptr;
+    return v.isPtr();
 }
 
 pub fn assert(v: Value) void {
@@ -18,7 +21,7 @@ pub fn assert(v: Value) void {
 // Foreign Pointers
 
 pub fn checkForeign(v: Value) bool {
-    return check(v) and v.ptr.foreign;
+    return check(v) and v.ptr.is_foreign;
 }
 
 pub fn assertForeign(v: Value) void {
@@ -28,151 +31,166 @@ pub fn assertForeign(v: Value) void {
     }
 }
 
-pub fn packForeign(int: u50) Value {
-    return .{ .fptr = .{int} };
+pub fn checkForeignRange(ptr: *anyopaque) bool {
+    const int = @intFromPtr(ptr);
+    return int <= std.math.maxInt(u50);
 }
 
-pub fn unpackForeign(v: Value) u64 {
+fn assertForeignRange(ptr: *anyopaque) void {
+    if (!checkForeignRange(ptr)) {
+        std.debug.print("foreign pointer out of range: {}\n", .{ptr});
+        @panic("foreign pointer out of range");
+    }
+}
+
+pub fn packForeign(ptr: *anyopaque) Value {
+    assertForeignRange(ptr);
+    const int: u50 = @intCast(@intFromPtr(ptr));
+    return .{ .fptr = .{ .value = int } };
+}
+
+pub fn unpackForeign(v: Value) *anyopaque {
     assertForeign(v);
-    return v.ptr.value.foreign;
+    return @ptrFromInt(v.fptr.value);
 }
 
 // Zisp Pointers
 
-pub fn checkZisp(v: Value) bool {
-    return check(v) and !v.ptr.foreign;
+fn _checkZisp(v: Value) bool {
+    return check(v) and !v.ptr.is_foreign;
 }
 
-pub fn assertZisp(v: Value) void {
-    if (!checkZisp(v)) {
+fn _assertZisp(v: Value) void {
+    if (!_checkZisp(v)) {
         v.dump();
         @panic("not zisp pointer");
     }
 }
 
 pub fn checkWeak(v: Value) bool {
-    return checkZisp(v) and v.ptr.weak;
+    return _checkZisp(v) and v.zptr.is_weak;
 }
 
 pub fn assertWeak(v: Value) void {
     if (!checkWeak(v)) {
         v.dump();
-        @panic("not weak zisp pointer");
+        @panic("not zisp weak pointer");
+    }
+}
+
+pub fn checkZisp(v: Value, tag: Tag) bool {
+    return _checkZisp(v) and unpack(v).@"1" == tag;
+}
+
+pub fn assertZisp(v: Value, tag: Tag) void {
+    if (!checkZisp(v, tag)) {
+        v.dump();
+        @panic("not zisp pointer or wrong tag");
     }
 }
 
-pub fn checkNormal(v: Value) bool {
-    return checkZisp(v) and !v.ptr.weak;
+pub fn checkStrong(v: Value) bool {
+    return _checkZisp(v) and !v.zptr.is_weak;
 }
 
-pub fn assertNormal(v: Value) void {
-    if (!checkNormal(v)) {
+pub fn assertStrong(v: Value) void {
+    if (!checkStrong(v)) {
         v.dump();
-        @panic("not normal zisp pointer");
+        @panic("not zisp strong pointer");
     }
 }
 
-pub fn packZisp(ptr: *anyopaque, tag: Tag, weak: bool) Value {
-    return .{ .ptr = .{
-        .value = tagPtr(ptr, tag),
-        .weak = weak,
+pub fn packZisp(ptr: [*]Bucket, tag: Tag, is_weak: bool) Value {
+    return .{ .zptr = .{
+        .tagged_value = tagPtr(ptr, tag),
+        .is_weak = is_weak,
     } };
 }
 
-pub fn pack(ptr: *anyopaque, tag: Tag) Value {
+pub fn pack(ptr: [*]Bucket, tag: Tag) Value {
     return packZisp(ptr, tag, false);
 }
 
-pub fn packWeak(ptr: *anyopaque, tag: Tag) Value {
+pub fn packWeak(ptr: [*]Bucket, tag: Tag) Value {
     return packZisp(ptr, tag, true);
 }
 
 // Unpacks weak as well; no need for a separate fn.
-pub fn unpack(v: Value) PtrAndTag {
-    assertZisp(v);
-    return untagPtr(v.ptr.value);
+pub fn unpack(v: Value) struct { [*]Bucket, Tag } {
+    _assertZisp(v);
+    return untagPtr(v.zptr.tagged_value);
 }
 
-// Weak pointers may be null.
-pub fn isNull(v: Value) bool {
-    assertWeak(v);
-    const ptr, _ = untagPtr(v.ptr.value);
-    return @intFromPtr(ptr) == 0;
+pub fn setWeakNull(v: *Value) void {
+    assertWeak(v.*);
+    v.zptr.tagged_value = 0;
 }
 
-pub fn tagPtr(ptr: *anyopaque, tag: Tag) u49 {
-    const int: u64 = @intFromPtr(ptr);
-    const untagged: u49 = @truncate(int);
-    return untagged << 1 | @intFromEnum(tag);
+pub fn isWeakNull(v: Value) bool {
+    assertWeak(v);
+    return v.zptr.tagged_value == 0;
 }
 
-pub const PtrAndTag = struct { *anyopaque, Tag };
+fn tagPtr(ptr: [*]Bucket, tag: Tag) u48 {
+    const int: usize = @intFromPtr(ptr);
+    const untagged: u48 = @intCast(int);
+    return untagged | @intFromEnum(tag);
+}
 
-pub fn untagPtr(tagged: u49) PtrAndTag {
-    const untagged: u49 = tagged >> 1 & 0xfffffffffff0;
-    const ptr: *anyopaque = @ptrFromInt(untagged);
-    const int: u4 = @truncate(tagged);
+fn untagPtr(tagged: u48) struct { [*]Bucket, Tag } {
+    const untagged: u48 = tagged & 0xfffffffffff8;
+    const ptr: [*]Bucket = @ptrFromInt(untagged);
+    const int: u3 = @truncate(tagged);
     const tag: Tag = @enumFromInt(int);
     return .{ ptr, tag };
 }
 
-pub const Tag = enum(u4) {
+pub const Tag = enum(u3) {
     /// 0. Strings / Symbols
     string,
     /// 1. Bignums / Ratnums
     number,
     /// 2. Pairs ([2]Value)
     pair,
-    /// 3. Vector, bytevector, etc.
-    array,
-    /// 4. Ordered hash table
-    table,
-    /// 5. String buffer
+    /// 3. Collections: Vector, table, etc.
+    coll,
+    /// 4. OOP: Classes, instances, etc.
+    oop,
+    /// 5. String buffers
     text,
-    /// 6. Class, interface, etc.
-    role,
-    /// 7. Instance, basically
-    actor,
-    /// 8. I/O Port
-    port,
-    /// 9. Procedure
+    /// 6. Procedures
     proc,
-    /// 10. Continuation
-    cont,
-    /// Other
-    other = 15,
+    /// 7. Others
+    other,
 };
 
 // Zisp API
 
 pub fn predForeign(v: Value) Value {
-    return Value.boole.pack(checkForeign(v));
+    return value.boole.pack(checkForeign(v));
 }
 
 pub fn makeWeak(v: Value) Value {
-    assertNormal(v);
+    assertStrong(v);
     var copy = v;
-    copy.ptr.weak = true;
+    copy.zptr.is_weak = true;
     return copy;
 }
 
 pub fn predWeak(v: Value) Value {
-    const isWeak = checkWeak(v);
-    return Value.boole.pack(isWeak);
+    return value.boole.pack(checkWeak(v));
 }
 
 pub fn predWeakNull(v: Value) Value {
-    assertWeak(v);
-    return Value.boole.pack(v.ptr.weak);
+    return value.boole.pack(isWeakNull(v));
 }
 
 pub fn getWeak(v: Value) Value {
-    assertWeak(v);
-    if (isNull(v)) {
-        return Value.boole.pack(false);
+    if (isWeakNull(v)) {
+        return value.boole.f;
     } else {
         var copy = v;
-        copy.ptr.weak = false;
+        copy.zptr.is_weak = false;
         return copy;
     }
 }
diff --git a/src/libzisp/value/sstr.zig b/src/libzisp/value/sstr.zig
index 55b3f8b..896b8d7 100644
--- a/src/libzisp/value/sstr.zig
+++ b/src/libzisp/value/sstr.zig
@@ -5,10 +5,7 @@ const Value = @import("../value.zig").Value;
 // Zig API
 
 pub fn check(v: Value) bool {
-    return v.isPacked() and
-        !v.sstr.fixnum and
-        !v.sstr.ptr and
-        v.sstr.tag == .sstr;
+    return v.isOther(.sstr);
 }
 
 pub fn assert(v: Value) void {
@@ -41,25 +38,22 @@ fn assertValidSstr(s: []const u8) void {
 
 // Different ways of doing the following have been tested, including manual
 // shifting and bit masking, but memcpy always wins easily according to our
-// micro-benchmarks, both under ReleaseSafe and under ReleaseFast.
+// micro-benchmarks, under both ReleaseSafe and ReleaseFast.
 
 pub fn pack(s: []const u8) Value {
     assertValidSstr(s);
-    var v = Value{ .sstr = .{ .value = 0 } };
-    const dest: [*]u8 = @ptrCast(&v.sstr.value);
+    var v = Value{ .sstr = .{ .string = 0 } };
+    const dest: [*]u8 = @ptrCast(&v.sstr.string);
     @memcpy(dest, s);
     return v;
 }
 
-// It's tempting to inline for here to eliminate the if statement or prevent
-// need of @truncate but all alternatives were a little slower.
-
 pub fn unpack(v: Value) struct { [6]u8, u3 } {
-    var s: [6]u8 = undefined;
-    const src: *const [6]u8 = @ptrCast(&v.sstr.value);
-    @memcpy(&s, src);
-    for (0..6) |i| {
-        if (s[i] == 0) return .{ s, @intCast(i) };
+    const s: [6]u8 = @bitCast(v.sstr.string);
+    inline for (0..6) |i| {
+        if (s[i] == 0) return .{ s, i };
     }
     return .{ s, 6 };
 }
+
+// No Zisp API for sstr specifically, since it's a string.  See string.zig.