Definition gfield_type (t: type) (f: ident) : type :=
match t with
| Tstruct _ fs => field_type f fs
| _ => Tint (* default return value *)
end.
Definition gfield_offset (t: type) (f: ident) : Z :=
match t with
| Tstruct _ fs => field_offset f fs
| _ => 0 (* default return value *)
end.
Fixpoint nested_field_type (t: type) (nf: list ident) : type :=
match nf with
| nil => t
| f :: nf0 => gfield_type (nested_field_type t nf0) f
end.
Fixpoint nested_field_offset (t: type) (nf: list ident) : Z :=
match nf with
| nil => 0
| f :: nf0 => nested_field_offset t nf0 + gfield_offset (nested_field_type t nf0) f
end.
Definition legal_field (t: type) (f: ident) :=
match t, f with
| Tstruct _ fs, i => in_members f fs
| _, _ => False
end.
Fixpoint legal_nested_field (t: type) (nf: list ident) : Prop :=
match nf with
| nil => True
| f :: nf0 => legal_nested_field t nf0 /\ legal_field (nested_field_type t nf0) f
end.
Fixpoint proj_struct (f : ident) (fs : fieldlist)
(v: reptype_fieldlist fs): reptype (field_type f fs) :=
match fs as fs_PAT
return reptype_fieldlist fs_PAT -> reptype (field_type f fs_PAT) with
| Fnil => fun _ => Vundef (* default return value *)
| Fcons (f', t) fs_tl =>
fun v =>
if ident_eq f f' as b
return reptype (if b then t else (field_type f fs_tl))
then fst v
else proj_struct f fs_tl (snd v)
end v.
Definition proj_gfield_reptype (t: type) (f: ident)
(v: reptype t): reptype (gfield_type t f) :=
match t as t_PAT
return (reptype t_PAT -> reptype (gfield_type t_PAT f))
with
| Tstruct _ fs => fun v => proj_struct f fs v
| _ => fun _ => Vundef
end v.
Fixpoint proj_reptype (t: type) (nf: list ident)
(v: reptype t) : reptype (nested_field_type t nf) :=
match nf as nf_PAT
return reptype (nested_field_type t nf_PAT)
with
| nil => v
| f :: nf0 => proj_gfield_reptype _ f (proj_reptype t nf0 v)
end.
Fixpoint upd_struct (f : ident) (fs : fieldlist) (v: reptype_fieldlist fs) (v0: reptype (field_type f fs)) : reptype_fieldlist fs :=
match fs as fs_PAT return (reptype_fieldlist fs_PAT -> reptype (field_type f fs_PAT) -> reptype_fieldlist fs_PAT) with
| Fnil => fun _ _ => tt (* default return value *)
| Fcons (f', t) fs_tl =>
fun v: reptype t * reptype_fieldlist fs_tl =>
if ident_eq f f' as b return reptype (if b then t else (field_type f fs_tl)) -> reptype t * reptype_fieldlist fs_tl
then fun v0 => (v0, snd v)
else fun v0 => (fst v, upd_struct f fs_tl (snd v) v0)
end v v0.
Definition upd_gfield_reptype t f (v: reptype t) (v0: reptype (gfield_type t f)) : reptype t :=
match t return (reptype t -> reptype (gfield_type t f) -> reptype t)
with
| Tstruct _ fs =>
fun v v0 => upd_struct f fs v v0
| _ => fun v _ => v
end v v0.
Fixpoint upd_reptype (t: type) (nf: list ident) (v: reptype t) (v0: reptype (nested_field_type t nf)): reptype t :=
match nf as nf_PAT
return reptype (nested_field_type t nf_PAT) -> reptype t
with
| nil => fun v0 => v0
| gf :: gfs0 => fun v0 => upd_reptype t gfs0 v (upd_gfield_reptype _ gf (proj_reptype t gfs0 v) v0)
end v0.

Lemma proj_upd_gfield_reptype_hit: forall t i v u,
legal_field t i ->
proj_gfield_reptype t i (upd_gfield_reptype t i u v) = v.