Work in progress

src/basic.jl

@@ -43,11 +43,7 @@ function Base.getindex(v::MemoryView, i::Integer)
     @inbounds ref[]
 end
 
-function Base.similar(
-    mem::MemoryView{T1, M},
-    ::Type{T2},
-    dims::Tuple{Int},
-) where {T1, T2, M}
+function Base.similar(::MemoryView{T1, M}, ::Type{T2}, dims::Tuple{Int}) where {T1, T2, M}
     len = Int(only(dims))::Int
     memory = Memory{T2}(undef, len)
     MemoryView{T2, M}(unsafe, memoryref(memory), len)
@@ -89,6 +85,23 @@ end
 Base.getindex(v::MemoryView, ::Colon) = v
 Base.view(v::MemoryView, idx::AbstractUnitRange) = v[idx]
 
+function truncate(mem::MemoryView, include_last::Integer)
+    lst = Int(include_last)::Int
+    @boundscheck if (lst % UInt) > length(mem) % UInt
+        throw(BoundsError(mem, lst))
+    end
+    typeof(mem)(unsafe, mem.ref, lst)
+end
+
+function truncate_start_nonempty(mem::MemoryView, from::Integer)
+    frm = Int(from)::Int
+    @boundscheck if ((frm - 1) % UInt) ≥ length(mem) % UInt
+        throw(BoundsError(mem, frm))
+    end
+    newref = @inbounds memoryref(mem.ref, frm)
+    typeof(mem)(unsafe, newref, length(mem) - frm + 1)
+end
+
 function Base.unsafe_copyto!(dst::MutableMemoryView{T}, src::MemoryView{T}) where {T}
     iszero(length(src)) && return dst
     @inbounds unsafe_copyto!(dst.ref, src.ref, length(src))
@@ -116,16 +129,6 @@ function Base.findnext(p::Function, mem::MemoryView, start::Integer)
     nothing
 end
 
-function Base.findprev(p::Function, mem::MemoryView, start::Integer)
-    i = Int(start)::Int
-    @boundscheck (i > length(mem) && throw(BoundsError(mem, i)))
-    @inbounds while i > 0
-        p(mem[i]) && return i
-        i -= 1
-    end
-    nothing
-end
-
 # The following two methods could be collapsed, but they aren't for two reasons:
 # * To prevent ambiguity with Base
 # * Because we DON'T want this code to run with MemoryView{Union{UInt8, Int8}}.
@@ -147,16 +150,17 @@ function Base.findnext(
     _findnext(mem, p.x, start)
 end
 
-@inline function _findnext(
+Base.@propagate_inbounds function _findnext(
     mem::MemoryView{T},
     byte::Union{T},
     start::Integer,
 ) where {T <: Union{UInt8, Int8}}
     start = Int(start)::Int
-    real_start = max(start, 1)
-    v = @inbounds ImmutableMemoryView(mem[real_start:end])
-    v_ind = @something memchr(v, byte) return nothing
-    v_ind + real_start - 1
+    @boundscheck(start < 1 && throw(BoundsError(mem, start)))
+    start > length(mem) && return nothing
+    im = @inbounds truncate_start_nonempty(ImmutableMemoryView(mem), start)
+    v_ind = @something memchr(im, byte) return nothing
+    v_ind + start - 1
 end
 
 function memchr(mem::ImmutableMemoryView{T}, byte::T) where {T <: Union{Int8, UInt8}}
@@ -172,6 +176,58 @@ function memchr(mem::ImmutableMemoryView{T}, byte::T) where {T <: Union{Int8, UI
     p == C_NULL ? nothing : (p - ptr) % Int + 1
 end
 
+function Base.findprev(p::Function, mem::MemoryView, start::Integer)
+    i = Int(start)::Int
+    @boundscheck (i > length(mem) && throw(BoundsError(mem, i)))
+    @inbounds while i > 0
+        p(mem[i]) && return i
+        i -= 1
+    end
+    nothing
+end
+
+function Base.findprev(
+    p::Base.Fix2{<:Union{typeof(==), typeof(isequal)}, UInt8},
+    mem::MemoryView{UInt8},
+    start::Integer,
+)
+    _findprev(mem, p.x, start)
+end
+
+function Base.findprev(
+    p::Base.Fix2{<:Union{typeof(==), typeof(isequal)}, Int8},
+    mem::MemoryView{Int8},
+    start::Integer,
+)
+    _findprev(mem, p.x, start)
+end
+
+Base.@propagate_inbounds function _findprev(
+    mem::MemoryView{T},
+    byte::Union{T},
+    start::Integer,
+) where {T <: Union{UInt8, Int8}}
+    start = Int(start)::Int
+    @boundscheck (start > length(mem) && throw(BoundsError(mem, start)))
+    start < 1 && return nothing
+    im = @inbounds truncate(ImmutableMemoryView(mem), start)
+    v_ind = @something memrchr(im, byte) return nothing
+    v_ind + start - 1
+end
+
+function memrchr(mem::ImmutableMemoryView{T}, byte::T) where {T <: Union{Int8, UInt8}}
+    isempty(mem) && return nothing
+    GC.@preserve mem begin
+        ptr = Ptr{UInt8}(pointer(mem))
+        p = @ccall memrchr(
+            ptr::Ptr{UInt8},
+            (byte % UInt8)::UInt8,
+            length(mem)::Int,
+        )::Ptr{Cvoid}
+    end
+    p == C_NULL ? nothing : (p - ptr) % Int + 1
+end
+
 const Bits =
     Union{Int8, UInt8, Int16, UInt16, Int32, UInt32, Int64, UInt64, Int128, UInt128, Char}