1	# Shared affine storage operations. Shape descriptors do not own another buffer.
2	import Base
3	import ./traversal_array.bend as Traversal
4	import ./proofs/storage_certificate.bend as Certificate
5	
6	type Buffer<-Element: Data> is Type:
7	  Buffer{storage: Array<Element>,length: Nat,certificate: Certificate.Certificate<Element,storage>}
8	
9	# Construction records evidence once, without visiting the owner or carrying
10	# certificates through the elementwise loop.
11	def allocate(~Element: Data,+depth: Nat,+value: Element,length: Nat) -> Buffer<Element>:
12	  Buffer{Array.new(Element,depth,value),length,Certificate.allocated(Element,depth,value)}
13	
14	def from_list(~Element: Data,+values: List<&2,Element>,+depth: Nat,+fallback: Element,length: Nat) -> Buffer<Element>:
15	  Buffer{Traversal.write_list(~Element,values,0,Array.new(Element,depth,fallback)),length,
16	    Certificate.write_list(~Element,values,0,Array.new(Element,depth,fallback),Certificate.allocated(Element,depth,fallback))}
17	
18	# The values written from the offset on; the certificate follows the storage.
19	def write_at(~Element: Data,+values: List<&2,Element>,+offset: U32,buffer: Buffer<Element>) -> Buffer<Element>:
20	  Buffer{storage,length,certificate} = buffer
21	  Buffer{Traversal.write_list(~Element,values,offset,storage),length,Certificate.write_list(~Element,values,offset,storage,certificate)}
22	
23	def capacity(~Element: Data,-storage: Array<Element>,certificate: Certificate.Certificate<Element,storage>) -> U32:
24	  Certificate.Certificate{depth,equation} = certificate
25	  U32.shln(1,depth)
26	
27	def bind_pair(~First: Type,~Second: Type,~Result: Type,pair: First & Second,body: First -> Second -> Result) -> Result:
28	  (first,second) = pair
29	  body(first,second)
30	
31	# At most 32 word steps, independent of the allocation length.
32	def bit_length(remaining_bits: Nat,+value: U32) -> Nat:
33	  match remaining_bits:
34	    case 0n: 0n
35	    case 1n+rest:
36	      Bool.pick(Nat,U32.is_zero(value),0n,1n+bit_length(rest,U32.shr(value)))
37	
38	# Positive requested lengths round to the next power-of-two capacity.
39	# The caller establishes the machine allocation limit before calling Array.new.
40	def allocation_depth(length: U32) -> Nat:
41	  bit_length(32n,(length - 1 : U32))
42	
43	# Shared runtime depth calculation. The tail call stops at the significant
44	# word bits; allocation_depth remains the arithmetic model used by proofs.
45	def significant_bits(remaining: Nat,+value: U32,+bits: Nat,zero: Bool) -> Nat:
46	  match remaining zero:
47	    case 0n zero: bits
48	    case 1n+rest True{}: bits
49	    case 1n+rest False{}:
50	      +next = U32.shr(value)
51	      significant_bits(rest,next,1n+bits,U32.is_zero(next))
52	
53	def depth_for(+count: U32) -> Nat:
54	  +value = (U32.max(count,1) - 1 : U32)
55	  significant_bits(32n,value,0n,U32.is_zero(value))
56	
57	def write_list(~Element: Data,values: List<&2,Element>,+offset: U32,storage: Array<Element>) -> Array<Element>:
58	  Traversal.write_list(~Element,values,offset,storage)
59	
60	def read_list(~Element: Data,count: Nat,+offset: U32,storage: Array<Element>) -> List<&2,Element>:
61	  match count:
62	    case 0n: []
63	    case 1n+rest:
64	      bind_pair(Array<Element>,Element,List<&2,Element>,Array.get(Element,storage,offset),storage => value =>
65	        value <> read_list(Element,rest,(offset + 1 : U32),storage))
66	
67	def read_when(~Element: Data,fallback: Element,valid: Bool,storage: Array<Element>,index: U32) -> Array<Element> & Element:
68	  match valid:
69	    case False{}: (storage,fallback)
70	    case True{}: Array.get(Element,storage,index)
71	
72	def read_sized(~Element: Data,fallback: Element,+index: U32,+logical_length: U32,sized: Array<Element> & U32) -> Array<Element> & Element:
73	  (storage,capacity) = sized
74	  read_when(Element,fallback,U32.is_lt(index,logical_length) && U32.is_le(logical_length,capacity),storage,index)
75	
76	def read_or(~Element: Data,fallback: Element,storage: Array<Element>,index: U32,logical_length: U32) -> Array<Element> & Element:
77	  read_sized(Element,fallback,index,logical_length,Array.size(Element,storage))