string.phi

{⟦
  org ↦ ⟦
    eolang ↦ ⟦
      string(as-bytes) ↦ ⟦
        φ ↦ ξ.as-bytes,
        length ↦ ⟦
          size ↦ ξ.ρ.as-bytes.size,
          pattern-one ↦ Φ̇.bytes(⟦ Δ ⤍ 80- ⟧),
          pattern-two ↦ Φ̇.bytes(⟦ Δ ⤍ E0- ⟧),
          pattern-three ↦ Φ̇.bytes(⟦ Δ ⤍ F0- ⟧),
          pattern-four ↦ Φ̇.bytes(⟦ Δ ⤍ F8- ⟧),
          result-one ↦ Φ̇.bytes(⟦ Δ ⤍ 00- ⟧),
          result-two ↦ Φ̇.bytes(⟦ Δ ⤍ C0- ⟧),
          result-three ↦ ξ.pattern-two,
          result-four ↦ ξ.pattern-three,
          φ ↦ ξ.size.eq(0).if(0, ξ.rec-length(0, 0)),
          increase-length(index, char-size, len) ↦ ⟦
            φ ↦ ξ.index.plus(ξ.char-size).gt(ξ.ρ.size).if(
              Φ̇.error(
                Φ̇.txt.sprintf(
                  "Expected %d byte character at %d index, but there are not enough bytes for it: %x",
                  Φ̇.tuple(
                    Φ̇.tuple(Φ̇.tuple(Φ̇.tuple.empty, ξ.char-size), ξ.index), ξ.ρ.ρ.as-bytes
                  )
                )
              ),
              ξ.ρ.rec-length(ξ.index.plus(ξ.char-size), ξ.len.plus(1))
            )
          ⟧,
          rec-length(index, accum) ↦ ⟦
            byte ↦ ξ.ρ.ρ.as-bytes.slice(ξ.index, 1),
            φ ↦ ξ.index.eq(ξ.ρ.size).if(
              ξ.accum,
              ξ.byte.and(ξ.ρ.pattern-one).eq(ξ.ρ.result-one).if(
                ξ.ρ.increase-length(ξ.index, 1, ξ.accum),
                ξ.byte.and(ξ.ρ.pattern-two).eq(ξ.ρ.result-two).if(
                  ξ.ρ.increase-length(ξ.index, 2, ξ.accum),
                  ξ.byte.and(ξ.ρ.pattern-three).eq(ξ.ρ.result-three).if(
                    ξ.ρ.increase-length(ξ.index, 3, ξ.accum),
                    ξ.byte.and(ξ.ρ.pattern-four).eq(ξ.ρ.result-four).if(
                      ξ.ρ.increase-length(ξ.index, 4, ξ.accum),
                      Φ̇.error(
                        Φ̇.txt.sprintf(
                          "Unknown byte format (%x), can't recognize character",
                          Φ̇.tuple(Φ̇.tuple.empty, ξ.byte)
                        )
                      )
                    )
                  )
                )
              )
            )
          ⟧
        ⟧,
        slice(start, len) ↦ ⟦
          start-bytes ↦ Φ̇.dataized(ξ.start).as-bytes,
          len-bytes ↦ Φ̇.dataized(ξ.len).as-bytes,
          num-start ↦ Φ̇.number(ξ.start-bytes),
          num-len ↦ Φ̇.number(ξ.len-bytes),
          size ↦ ξ.ρ.as-bytes.size,
          end ↦ ξ.num-start.plus(ξ.num-len),
          pattern-one ↦ Φ̇.bytes(⟦ Δ ⤍ 80- ⟧),
          pattern-two ↦ Φ̇.bytes(⟦ Δ ⤍ E0- ⟧),
          pattern-three ↦ Φ̇.bytes(⟦ Δ ⤍ F0- ⟧),
          pattern-four ↦ Φ̇.bytes(⟦ Δ ⤍ F8- ⟧),
          result-one ↦ Φ̇.bytes(⟦ Δ ⤍ 00- ⟧),
          result-two ↦ Φ̇.bytes(⟦ Δ ⤍ C0- ⟧),
          result-three ↦ ξ.pattern-two,
          result-four ↦ ξ.pattern-three,
          bts-start ↦ Φ̇.dataized(
            ξ.rec-index(
              0,
              0,
              ξ.num-start,
              Φ̇.txt.sprintf(
                "Start index (%d) is out of string bounds",
                Φ̇.tuple(Φ̇.tuple.empty, ξ.num-start)
              )
            )
          ).as-bytes,
          bts-length ↦ ξ.rec-index(
            Φ̇.number(ξ.bts-start),
            0,
            ξ.num-len,
            Φ̇.txt.sprintf(
              "Start index + length to slice (%d) is out of string bounds",
              Φ̇.tuple(Φ̇.tuple.empty, ξ.num-start.plus(ξ.num-len))
            )
          ).minus(ξ.bts-start),
          φ ↦ ξ.num-start.lt(0).if(
            Φ̇.error(
              Φ̇.txt.sprintf(
                "Start index must be >= 0, but was %d", Φ̇.tuple(Φ̇.tuple.empty, ξ.num-start)
              )
            ),
            ξ.num-len.lt(0).if(
              Φ̇.error(
                Φ̇.txt.sprintf(
                  "Length to slice must be >= 0, but was %d", Φ̇.tuple(Φ̇.tuple.empty, ξ.num-len)
                )
              ),
              ξ.num-len.eq(0).if(
                "", Φ̇.string(ξ.ρ.as-bytes.slice(ξ.bts-start, ξ.bts-length))
              )
            )
          ),
          increase(index, char-size, accum, result, cause) ↦ ⟦
            φ ↦ ξ.index.plus(ξ.char-size).gt(ξ.ρ.size).if(
              Φ̇.error(
                Φ̇.txt.sprintf(
                  "Expected %d byte character at %d index, but there are not enough bytes for it: %x",
                  Φ̇.tuple(
                    Φ̇.tuple(Φ̇.tuple(Φ̇.tuple.empty, ξ.char-size), ξ.index), ξ.ρ.ρ.as-bytes
                  )
                )
              ),
              ξ.ρ.rec-index(
                ξ.index.plus(ξ.char-size), ξ.accum.plus(1), ξ.result, ξ.cause
              )
            )
          ⟧,
          rec-index(index, accum, result, cause) ↦ ⟦
            byte ↦ ξ.ρ.ρ.as-bytes.slice(ξ.index, 1),
            φ ↦ ξ.accum.eq(ξ.result).if(
              ξ.index,
              ξ.index.eq(ξ.ρ.size).if(
                Φ̇.error(ξ.cause),
                ξ.byte.and(ξ.ρ.pattern-one).eq(ξ.ρ.result-one).if(
                  ξ.ρ.increase(ξ.index, 1, ξ.accum, ξ.result, ξ.cause),
                  ξ.byte.and(ξ.ρ.pattern-two).eq(ξ.ρ.result-two).if(
                    ξ.ρ.increase(ξ.index, 2, ξ.accum, ξ.result, ξ.cause),
                    ξ.byte.and(ξ.ρ.pattern-three).eq(ξ.ρ.result-three).if(
                      ξ.ρ.increase(ξ.index, 3, ξ.accum, ξ.result, ξ.cause),
                      ξ.byte.and(ξ.ρ.pattern-four).eq(ξ.ρ.result-four).if(
                        ξ.ρ.increase(ξ.index, 4, ξ.accum, ξ.result, ξ.cause),
                        Φ̇.error(
                          Φ̇.txt.sprintf(
                            "Unknown byte format (%x), can't recognize character",
                            Φ̇.tuple(Φ̇.tuple.empty, ξ.byte)
                          )
                        )
                      )
                    )
                  )
                )
              )
            )
          ⟧
        ⟧
      ⟧,
      λ ⤍ Package
    ⟧,
    λ ⤍ Package
  ⟧
⟧}