/-
MetadataSurfaceComputation.lean — Metadata Surface Computation with No Payload Transmission

This module formalizes metadata surface computation: the payload never moves,
only the metadata surface is exposed.

Per AGENTS.md §1.6: No proof placeholders in committed code.
Per AGENTS.md §1.4: Uses Q16_16 fixed-point for hardware-native computation.
Per AGENTS.md §2: PascalCase types, camelCase functions.
Per AGENTS.md §4: All defs must have eval witnesses or theorems.

Reference: ChatGPT conversation on Layer 3 Crypto Networks (2026-04-27)
-/

import Std
import Mathlib.Data.Nat.Basic
import Mathlib.Tactic

namespace Semantics.MetadataSurfaceComputation

/-- A local object that stays local (payload never transmitted) -/
structure LocalObject where
  id : String
  type : String
  payload : String  -- Never transmitted
  deriving Repr, Inhabited

/-- A metadata surface event (only surface deformation, not payload) -/
structure MetadataSurface where
  objectId : String
  routeId : Nat
  shellId : Nat
  timestampBucket : Nat
  routeState : String
  pressure : Nat
  gate : String
  commitment : String
  deriving Repr, Inhabited

/-- Expose metadata surface from a local object -/
def exposeMetadataSurface (obj : LocalObject) : MetadataSurface :=
  {
    objectId := obj.id,
    routeId := 0,
    shellId := 0,
    timestampBucket := 0,
    routeState := "active",
    pressure := 0,
    gate := "open",
    commitment := "pending"
  }

/-- A shell that computes from observable surface geometry -/
structure MetadataShell where
  id : Nat
  surface : MetadataSurface
  deriving Repr, Inhabited

/-- Compute result from metadata surface (payload never accessed) -/
def computeFromSurface (shell : MetadataShell) : Nat :=
  shell.surface.pressure + shell.surface.shellId

/-- A receipt encoding the result as metadata -/
structure MetadataReceipt where
  inputHash : String
  resultHash : String
  shellId : Nat
  timestamp : Nat
  deriving Repr, Inhabited

/-- Generate receipt from shell computation -/
def generateReceipt (shell : MetadataShell) : MetadataReceipt :=
  {
    inputHash := shell.surface.objectId,
    resultHash := s!"${computeFromSurface shell}",
    shellId := shell.id,
    timestamp := 0
  }

/-- Core receipt law: the generated receipt commits to the object id, not payload bytes. -/
theorem payloadReceiptUsesOnlyObjectId (obj : LocalObject) (shell : MetadataShell) :
  let surface := exposeMetadataSurface obj
  let receipt := generateReceipt { shell with surface := surface }
  receipt.inputHash = obj.id := by
  rfl

theorem exposedSurfaceForgetsPayload (obj : LocalObject) :
    (exposeMetadataSurface obj).objectId = obj.id := by
  rfl

#eval (exposeMetadataSurface { id := "obj-1", type := "tensor", payload := "local" }).objectId
#eval (generateReceipt { id := 7, surface := exposeMetadataSurface { id := "obj-1", type := "tensor", payload := "local" } }).shellId

end Semantics.MetadataSurfaceComputation