feat: EIP-7843 — SLOTNUM opcode (Amsterdam) (#10)

* feat: EIP-7843 — SLOTNUM opcode (Amsterdam)

Adds SLOTNUM (0x4B): pushes the beacon chain slot number from the block
environment. Gas: G_BASE (2). Adds slot_number field to BlockEnv and
BlockEnvBuilder (defaults to 0).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(eip-7843): make slot_number optional everywhere per review feedback

- `BlockEnv.slot_number`: `u64` → `?u64`; default is `null` (absent on
  pre-Amsterdam / non-beacon chains)
- `BlockEnvBuilder`: add `setSlotNumber(?u64)`; propagate `slot_number`
  through all existing setter methods (previously silently dropped)
- `Host.slotNumber()`: returns `?u64`
- `opSlotnum`: halt with `invalid_opcode` when slot number is absent
- `spec_test/runner.zig`: initialize `slot_number` to `null`

Addresses review comment from @garyschulte on PR #10.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Gabriel-Trintinalia

src/bytecode/main.zig

@@ -225,6 +225,7 @@ pub const SELFBALANCE: u8 = 0x47;
 pub const BASEFEE: u8 = 0x48;
 pub const BLOBHASH: u8 = 0x49;
 pub const BLOBBASEFEE: u8 = 0x4A;
+pub const SLOTNUM: u8 = 0x4B;
 pub const POP: u8 = 0x50;
 pub const MLOAD: u8 = 0x51;
 pub const MSTORE: u8 = 0x52;
@@ -387,6 +388,7 @@ pub const OPCODE_INFO: [256]?OpCodeInfo = blk: {
     map[BASEFEE] = OpCodeInfo{ .name = "BASEFEE", .inputs = 0, .outputs = 1, .immediate_size = 0, .terminating = false };
     map[BLOBHASH] = OpCodeInfo{ .name = "BLOBHASH", .inputs = 1, .outputs = 1, .immediate_size = 0, .terminating = false };
     map[BLOBBASEFEE] = OpCodeInfo{ .name = "BLOBBASEFEE", .inputs = 0, .outputs = 1, .immediate_size = 0, .terminating = false };
+    map[SLOTNUM] = OpCodeInfo{ .name = "SLOTNUM", .inputs = 0, .outputs = 1, .immediate_size = 0, .terminating = false };
 
     // Stack operations
     map[POP] = OpCodeInfo{ .name = "POP", .inputs = 1, .outputs = 0, .immediate_size = 0, .terminating = false };

src/context/block.zig

@@ -29,6 +29,11 @@ pub const BlockEnv = struct {
     ///
     /// Incorporated as part of the Cancun upgrade via EIP-4844.
     blob_excess_gas_and_price: ?BlobExcessGasAndPrice,
+    /// Beacon chain slot number (EIP-7843, Amsterdam+).
+    ///
+    /// `null` when the block header does not carry a slot number (pre-Amsterdam or
+    /// non-beacon chains).  `SLOTNUM` halts with `invalid_opcode` in that case.
+    slot_number: ?u64,
 
     pub fn default() BlockEnv {
         return .{
@@ -40,6 +45,7 @@ pub const BlockEnv = struct {
             .difficulty = @as(primitives.U256, 0),
             .prevrandao = null,
             .blob_excess_gas_and_price = BlobExcessGasAndPrice.new(0, primitives.BLOB_BASE_FEE_UPDATE_FRACTION_PRAGUE),
+            .slot_number = null,
         };
     }
 
@@ -143,6 +149,7 @@ pub const BlockEnvBuilder = struct {
     difficulty: ?primitives.U256,
     prevrandao: ?primitives.Hash,
     blob_excess_gas_and_price: ?BlobExcessGasAndPrice,
+    slot_number: ?u64,
 
     pub fn new() BlockEnvBuilder {
         return .{
@@ -154,6 +161,7 @@ pub const BlockEnvBuilder = struct {
             .difficulty = null,
             .prevrandao = null,
             .blob_excess_gas_and_price = null,
+            .slot_number = null,
         };
     }
 
@@ -167,6 +175,7 @@ pub const BlockEnvBuilder = struct {
             .difficulty = self.difficulty,
             .prevrandao = self.prevrandao,
             .blob_excess_gas_and_price = self.blob_excess_gas_and_price,
+            .slot_number = self.slot_number,
         };
     }
 
@@ -180,6 +189,7 @@ pub const BlockEnvBuilder = struct {
             .difficulty = self.difficulty,
             .prevrandao = self.prevrandao,
             .blob_excess_gas_and_price = self.blob_excess_gas_and_price,
+            .slot_number = self.slot_number,
         };
     }
 
@@ -193,6 +203,7 @@ pub const BlockEnvBuilder = struct {
             .difficulty = self.difficulty,
             .prevrandao = self.prevrandao,
             .blob_excess_gas_and_price = self.blob_excess_gas_and_price,
+            .slot_number = self.slot_number,
         };
     }
 
@@ -206,6 +217,7 @@ pub const BlockEnvBuilder = struct {
             .difficulty = self.difficulty,
             .prevrandao = self.prevrandao,
             .blob_excess_gas_and_price = self.blob_excess_gas_and_price,
+            .slot_number = self.slot_number,
         };
     }
 
@@ -219,6 +231,7 @@ pub const BlockEnvBuilder = struct {
             .difficulty = self.difficulty,
             .prevrandao = self.prevrandao,
             .blob_excess_gas_and_price = self.blob_excess_gas_and_price,
+            .slot_number = self.slot_number,
         };
     }
 
@@ -232,6 +245,7 @@ pub const BlockEnvBuilder = struct {
             .difficulty = difficulty,
             .prevrandao = self.prevrandao,
             .blob_excess_gas_and_price = self.blob_excess_gas_and_price,
+            .slot_number = self.slot_number,
         };
     }
 
@@ -245,6 +259,7 @@ pub const BlockEnvBuilder = struct {
             .difficulty = self.difficulty,
             .prevrandao = prevrandao,
             .blob_excess_gas_and_price = self.blob_excess_gas_and_price,
+            .slot_number = self.slot_number,
         };
     }
 
@@ -258,6 +273,21 @@ pub const BlockEnvBuilder = struct {
             .difficulty = self.difficulty,
             .prevrandao = self.prevrandao,
             .blob_excess_gas_and_price = blob_excess_gas_and_price,
+            .slot_number = self.slot_number,
+        };
+    }
+
+    pub fn setSlotNumber(self: BlockEnvBuilder, slot_number: ?u64) BlockEnvBuilder {
+        return .{
+            .number = self.number,
+            .beneficiary = self.beneficiary,
+            .timestamp = self.timestamp,
+            .gas_limit = self.gas_limit,
+            .basefee = self.basefee,
+            .difficulty = self.difficulty,
+            .prevrandao = self.prevrandao,
+            .blob_excess_gas_and_price = self.blob_excess_gas_and_price,
+            .slot_number = slot_number,
         };
     }
 
@@ -271,6 +301,7 @@ pub const BlockEnvBuilder = struct {
             .difficulty = self.difficulty orelse @as(primitives.U256, 0),
             .prevrandao = self.prevrandao,
             .blob_excess_gas_and_price = self.blob_excess_gas_and_price orelse BlobExcessGasAndPrice.new(0, primitives.BLOB_BASE_FEE_UPDATE_FRACTION_PRAGUE),
+            .slot_number = self.slot_number,
         };
     }
 };

src/interpreter/host.zig

@@ -163,6 +163,10 @@ pub const Host = struct {
         return self.ctx.blockHash(number);
     }
 
+    pub fn slotNumber(self: *Host) ?u64 {
+        return self.ctx.block.slot_number;
+    }
+
     // -----------------------------------------------------------------------
     // Account state access (via journaled_state)
     // -----------------------------------------------------------------------

src/interpreter/opcodes/environment.zig

@@ -512,3 +512,25 @@ pub fn opBlobbasefee(ctx: *InstructionContext) void {
     }
     stack.pushUnsafe(@as(primitives.U256, h.blobBasefee()));
 }
+
+/// SLOTNUM (0x4B): Push the beacon chain slot number (EIP-7843, Amsterdam+).
+/// Stack: [] -> [slotNumber]   Gas: 2 (G_BASE, dispatch)
+///
+/// Halts with `invalid_opcode` if no host is present or if the block environment
+/// does not carry a slot number (e.g. pre-Amsterdam or non-beacon execution).
+pub fn opSlotnum(ctx: *InstructionContext) void {
+    const h = ctx.host orelse {
+        ctx.interpreter.halt(.invalid_opcode);
+        return;
+    };
+    const slot = h.slotNumber() orelse {
+        ctx.interpreter.halt(.invalid_opcode);
+        return;
+    };
+    const stack = &ctx.interpreter.stack;
+    if (!stack.hasSpace(1)) {
+        ctx.interpreter.halt(.stack_overflow);
+        return;
+    }
+    stack.pushUnsafe(@as(primitives.U256, slot));
+}

src/interpreter/opcodes/main.zig

@@ -85,6 +85,7 @@ pub const opChainid = environment.opChainid;
 pub const opBasefee = environment.opBasefee;
 pub const opBlobhash = environment.opBlobhash;
 pub const opBlobbasefee = environment.opBlobbasefee;
+pub const opSlotnum = environment.opSlotnum;
 
 // Host-requiring opcodes (account state, storage, logs, selfdestruct)
 pub const host_ops = @import("host_ops.zig");

src/interpreter/protocol_schedule.zig

@@ -49,6 +49,7 @@ pub fn makeInstructionTable(spec: primitives.SpecId) InstructionTable {
     if (primitives.isEnabledIn(spec, .shanghai)) applyShanghaiChanges(&table);
     if (primitives.isEnabledIn(spec, .cancun)) applyCancunChanges(&table);
     if (primitives.isEnabledIn(spec, .osaka)) applyOsakaChanges(&table);
+    if (primitives.isEnabledIn(spec, .amsterdam)) applyAmsterdamChanges(&table);
 
     return table;
 }
@@ -254,6 +255,11 @@ fn applyOsakaChanges(table: *InstructionTable) void {
     table[bytecode_mod.CLZ] = entry(opcodes.opClz, gas_costs.G_LOW);
 }
 
+fn applyAmsterdamChanges(table: *InstructionTable) void {
+    // EIP-7843: SLOTNUM opcode — push beacon chain slot number
+    table[bytecode_mod.SLOTNUM] = entry(opcodes.opSlotnum, gas_costs.G_BASE);
+}
+
 // ---------------------------------------------------------------------------
 // Precompile set construction
 // ---------------------------------------------------------------------------

src/spec_test/runner.zig

@@ -259,6 +259,7 @@ pub fn runTestCase(tc: types.TestCase, allocator: std.mem.Allocator) TestOutcome
         .difficulty = u256FromBeBytes(tc.block_difficulty),
         .prevrandao = tc.prevrandao,
         .blob_excess_gas_and_price = blob_excess_gas_and_price,
+        .slot_number = null,
     });
 
     // Build access list for TxEnv