blurhash.bqn

c83 ← "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz#$%*+,-.:;=?@[]^_{|}~"
Base83 ← ⊏⟜c83∘{0:""; (𝕊⌊𝕩÷83)∾83|𝕩}
Base83Pad ← {
  s ← Base83𝕩
  s∾˜'0'⥊˜𝕨-≠s
}
Base83dec ← {
  xs ← c83⊐𝕩
  +´xs×⌽83⋆↕≠xs
}

DCT2d ← {
  s ← ≢𝕩
  basis ← ×´¨ •math.Cos π×𝕨⊸ר÷⟜s¨↕s
  ÷⟜(×´s)+´⥊basis×𝕩
}

Srgb2lin ← {
  u ← 𝕩÷255
  {u≤0.04045 ? u×12.92;
  ((u+0.055)÷1.055)⋆2.4}
}
Lin2srgb ← {
  𝕩≤0.0031308 ? ⌊0.5+12.92×255×0⌈𝕩;
  ⌊0.5+255×0.055-˜1.055×(𝕩⌊1)⋆÷2.4
}
Ac2ints ← {
  norm ← 166÷˜1+𝕨
  0⌈18⌊⌊9.5+9×(××⋆⟜0.5∘|)(𝕩÷norm)
}

EncodeLinear ⇐ {comps 𝕊 img:
  cs ← (⥊⍉↕comps) DCT2d⌜ <˘img

  dc ← ⊏cs
  ac ← 2×1↓cs

  c ← 1 Base83Pad +´1‿9×comps-1
  max ← ⌈´|⥊ac
  quant ← 0⌈82⌊⌊¯0.5+max×166
  q ← 1 Base83Pad quant
  a ← 4 Base83Pad +´(⌽256⊸⋆↕3) × Lin2srgb¨ dc
  d ← 2⊸Base83Pad¨ +˝(⌽19⊸⋆↕3) × ⍉ quant Ac2ints ac
  c∾q∾a∾∾´d
}
Encode ⇐ {
  𝕨 EncodeLinear Srgb2lin¨ 𝕩
}

Decomp ← {𝕨|⌊(𝕨⋆⌽↕3)÷˜𝕩}
DecodeLinear ⇐ {w‿h 𝕊 hash:
  x‿y ← ⌊1+9(|∾÷˜) Base83dec ⊑hash
  quant ← Base83dec 1⊑hash
  max ← 166÷˜1+quant
  dc ← Srgb2lin¨ 256 Decomp Base83dec 4↑2↓hash
  ac ← max×(××⋆⟜2∘|)9÷˜9-˜ 19 Decomp˘ Base83dec˘∘‿2⥊6↓hash
  comps ← (⥊⍉↕x‿y)∾˘dc∾ac
  img ← +˝>(<˘comps){⟨c,d⟩‿r‿g‿b 𝕊 x‿y:
    basis ← ×´•math.Cos π×c‿d×x‿y÷w‿h
    basis×r‿g‿b
  }⌜↕w‿h
  2‿0‿1⍉⁼img
}
Decode ⇐ {Lin2srgb¨ 𝕨 DecodeLinear 𝕩}