diff --git a/src/bam/bai.jl b/src/bam/bai.jl
index a017ca92..6237979f 100644
--- a/src/bam/bai.jl
+++ b/src/bam/bai.jl
@@ -33,6 +33,11 @@ function BAI(input::IO)
     return read_bai(input)
 end
 
+function findbai(filepath::AbstractString)
+    baipath = string(filepath, ".bai")
+    return isfile(baipath) ? Nullable(BAI(baipath)) : Nullable{BAI}()
+end
+
 # Read a BAI object from `input`.
 function read_bai(input::IO)
     # check magic bytes
diff --git a/src/bam/overlap.jl b/src/bam/overlap.jl
index 378fc14b..54904e86 100644
--- a/src/bam/overlap.jl
+++ b/src/bam/overlap.jl
@@ -31,7 +31,10 @@ end
 # Iterator
 # --------
 
-mutable struct OverlapIteratorState
+mutable struct OverlapIteratorState{S}
+    # reader's state
+    readerstate::ReaderState{S,Record}
+
     # reference index
     refindex::Int
 
@@ -40,30 +43,31 @@ mutable struct OverlapIteratorState
 
     # current chunk index
     chunkid::Int
-
-    # pre-allocated record
-    record::Record
 end
 
 function Base.start(iter::OverlapIterator)
-    refindex = findfirst(iter.reader.refseqnames, iter.refname)
+    readerstate = ReaderState(iter.reader)
+    reader = readerstate.reader
+    refindex = findfirst(reader.refseqnames, iter.refname)
     if refindex == 0
         throw(ArgumentError("sequence name $(iter.refname) is not found in the header"))
     end
-    @assert !isnull(iter.reader.index)
-    chunks = GenomicFeatures.Indexes.overlapchunks(get(iter.reader.index).index, refindex, iter.interval)
+    @assert !isnull(reader.index)
+    chunks = GenomicFeatures.Indexes.overlapchunks(get(reader.index).index, refindex, iter.interval)
     if !isempty(chunks)
-        seek(iter.reader, first(chunks).start)
+        seek(reader.input, first(chunks).start)
     end
-    return OverlapIteratorState(refindex, chunks, 1, Record())
+    return OverlapIteratorState(readerstate, refindex, chunks, 1)
 end
 
 function Base.done(iter::OverlapIterator, state)
+    reader = state.readerstate.reader
+    record = state.readerstate.record
     while state.chunkid ≤ endof(state.chunks)
         chunk = state.chunks[state.chunkid]
-        while BGZFStreams.virtualoffset(iter.reader.stream) < chunk.stop
-            read!(iter.reader, state.record)
-            c = compare_intervals(state.record, (state.refindex, iter.interval))
+        while BGZFStreams.virtualoffset(reader.input) < chunk.stop
+            read!(reader, record)
+            c = compare_intervals(record, (state.refindex, iter.interval))
             if c == 0
                 # overlapping
                 return false
@@ -74,14 +78,14 @@ function Base.done(iter::OverlapIterator, state)
         end
         state.chunkid += 1
         if state.chunkid ≤ endof(state.chunks)
-            seek(iter.reader, state.chunks[state.chunkid].start)
+            seek(reader.input, state.chunks[state.chunkid].start)
         end
     end
     return true
 end
 
 function Base.next(::OverlapIterator, state)
-    return copy(state.record), state
+    return copy(state.readerstate.record), state
 end
 
 function compare_intervals(record::Record, interval::Tuple{Int,UnitRange{Int}})
diff --git a/src/bam/reader.jl b/src/bam/reader.jl
index e7b08f47..bfc39127 100644
--- a/src/bam/reader.jl
+++ b/src/bam/reader.jl
@@ -2,47 +2,100 @@
 # ==========
 
 """
-    BAM.Reader(input::IO; index=nothing)
+    BAM.Reader(input; index=nothing)
 
 Create a data reader of the BAM file format.
 
 # Arguments
-* `input`: data source
-* `index=nothing`: filepath to a random access index (currently *bai* is Supported)
+* `input`: data source (filepath or readable `IO` object)
+* `index=nothing`: filepath to a random access index (currently *bai* is supported)
 """
-mutable struct Reader{T} <: Bio.IO.AbstractReader
-    stream::BGZFStreams.BGZFStream{T}
+mutable struct Reader{T<:Union{String,BGZFStreams.BGZFStream}} <: Bio.IO.AbstractReader
+    # data source
+    input::T
+
+    # BAM index
+    index::Nullable{BAI}
+
+    # header data
     header::SAM.Header
-    start_offset::BGZFStreams.VirtualOffset
     refseqnames::Vector{String}
     refseqlens::Vector{Int}
-    index::Nullable{BAI}
 end
 
-function Base.eltype{T}(::Type{Reader{T}})
-    return Record
-end
+function Reader(input::BGZFStreams.BGZFStream; index=nothing)
+    if index == nothing
+        index = Nullable{BAI}()
+    elseif index isa BAI
+        index = Nullable(index)
+    elseif index isa AbstractString
+        index = Nullable(BAI(index))
+    elseif index isa Nullable{BAI}
+        # ok
+    else
+        error("unrecognizable index argument: $(typeof(index))")
+    end
 
-function Bio.IO.stream(reader::Reader)
-    return reader.stream
+    # magic bytes
+    B = read(input, UInt8)
+    A = read(input, UInt8)
+    M = read(input, UInt8)
+    x = read(input, UInt8)
+    if B != UInt8('B') || A != UInt8('A') || M != UInt8('M') || x != 0x01
+        error("input was not a valid BAM file")
+    end
+
+    # SAM header
+    textlen = read(input, Int32)
+    samreader = SAM.Reader(IOBuffer(read(input, UInt8, textlen)))
+
+    # reference sequences
+    refseqnames = String[]
+    refseqlens = Int[]
+    n_refs = read(input, Int32)
+    for _ in 1:n_refs
+        namelen = read(input, Int32)
+        data = read(input, UInt8, namelen)
+        seqname = unsafe_string(pointer(data))
+        seqlen = read(input, Int32)
+        push!(refseqnames, seqname)
+        push!(refseqlens, seqlen)
+    end
+
+    return Reader(input, index, samreader.header, refseqnames, refseqlens)
 end
 
 function Reader(input::IO; index=nothing)
-    if isa(index, AbstractString)
-        index = BAI(index)
-    else
-        if index != nothing
-            error("unrecognizable index argument")
+    return Reader(BGZFStreams.BGZFStream(input), index=index)
+end
+
+function Reader(filepath::AbstractString; index=:auto)
+    if index isa Symbol
+        if index == :auto
+            index = findbai(filepath)
+        else
+            throw(ArgumentError("invalid index: ':$(index)'"))
         end
+    elseif index isa AbstractString
+        index = BAI(index)
     end
-    reader = init_bam_reader(input)
-    reader.index = index
-    return reader
+    return Reader(filepath, index, SAM.Header(), String[], Int[])
+end
+
+function Base.open(reader::Reader{String})
+    return Reader(open(reader.input), index=reader.index)
+end
+
+function Base.eltype{T}(::Type{Reader{T}})
+    return Record
+end
+
+function Bio.IO.stream(reader::Reader)
+    return reader.input
 end
 
 function Base.show(io::IO, reader::Reader)
-    println(io, summary(reader), ":")
-      print(io, "  number of contigs: ", length(reader.refseqnames))
+    print(io, summary(reader), "(<input=", repr(reader.input), ">)")
 end
 
 """
@@ -70,81 +123,36 @@ function Bio.header(reader::Reader)
     return header(reader)
 end
 
-function Base.seek(reader::Reader, voffset::BGZFStreams.VirtualOffset)
-    seek(reader.stream, voffset)
+#function Base.seek(reader::Reader, voffset::BGZFStreams.VirtualOffset)
+#    seek(reader.stream, voffset)
+#end
+#
+#function Base.seekstart(reader::Reader)
+#    seek(reader.stream, reader.start_offset)
+#end
+
+struct ReaderState{S,T}
+    reader::S
+    record::T
 end
 
-function Base.seekstart(reader::Reader)
-    seek(reader.stream, reader.start_offset)
+function ReaderState(reader::Reader{<:BGZFStreams.BGZFStream})
+    return ReaderState(reader, Record())
 end
 
-function Base.start(reader::Reader)
-    return Record()
+function ReaderState(reader::Reader{String})
+    return ReaderState(open(reader), Record())
 end
 
-function Base.done(reader::Reader, rec)
-    return eof(reader)
+function Base.start(reader::Reader)
+    return ReaderState(reader)
 end
 
-function Base.next(reader::Reader, rec)
-    read!(reader, rec)
-    return copy(rec), rec
+function Base.done(::Reader, state)
+    return eof(state.reader.input)
 end
 
-# Initialize a BAM reader by reading the header section.
-function init_bam_reader(input::BGZFStreams.BGZFStream)
-    # magic bytes
-    B = read(input, UInt8)
-    A = read(input, UInt8)
-    M = read(input, UInt8)
-    x = read(input, UInt8)
-    if B != UInt8('B') || A != UInt8('A') || M != UInt8('M') || x != 0x01
-        error("input was not a valid BAM file")
-    end
-
-    # SAM header
-    textlen = read(input, Int32)
-    samreader = SAM.Reader(IOBuffer(read(input, UInt8, textlen)))
-
-    # reference sequences
-    refseqnames = String[]
-    refseqlens = Int[]
-    n_refs = read(input, Int32)
-    for _ in 1:n_refs
-        namelen = read(input, Int32)
-        data = read(input, UInt8, namelen)
-        seqname = unsafe_string(pointer(data))
-        seqlen = read(input, Int32)
-        push!(refseqnames, seqname)
-        push!(refseqlens, seqlen)
-    end
-
-    voffset = isa(input.io, Pipe) ?
-        BGZFStreams.VirtualOffset(0, 0) :
-        BGZFStreams.virtualoffset(input)
-    return Reader(
-        input,
-        samreader.header,
-        voffset,
-        refseqnames,
-        refseqlens,
-        Nullable{BAI}())
-end
-
-function init_bam_reader(input::IO)
-    return init_bam_reader(BGZFStreams.BGZFStream(input))
-end
-
-function _read!(reader::Reader, record)
-    unsafe_read(
-        reader.stream,
-        pointer_from_objref(record),
-        FIXED_FIELDS_BYTES)
-    dsize = data_size(record)
-    if length(record.data) < dsize
-        resize!(record.data, dsize)
-    end
-    unsafe_read(reader.stream, pointer(record.data), dsize)
-    record.reader = reader
-    return record
+function Base.next(::Reader, state)
+    read!(state.reader, state.record)
+    return copy(state.record), state
 end
diff --git a/src/bam/record.jl b/src/bam/record.jl
index 965eeb28..69c23324 100644
--- a/src/bam/record.jl
+++ b/src/bam/record.jl
@@ -86,7 +86,17 @@ function Base.show(io::IO, record::Record)
 end
 
 function Base.read!(reader::Reader, record::Record)
-    return _read!(reader, record)
+    unsafe_read(
+        reader.input,
+        pointer_from_objref(record),
+        FIXED_FIELDS_BYTES)
+    dsize = data_size(record)
+    if length(record.data) < dsize
+        resize!(record.data, dsize)
+    end
+    unsafe_read(reader.input, pointer(record.data), dsize)
+    record.reader = reader
+    return record
 end
 
 
diff --git a/transcript_depth.jl b/transcript_depth.jl
new file mode 100644
index 00000000..fc2a55b6
--- /dev/null
+++ b/transcript_depth.jl
@@ -0,0 +1,68 @@
+@everywhere begin
+    using BioAlignments
+    using GenomicFeatures
+
+    # The main algorithm.
+    function compute_depth(reader, interval)
+        range = interval.first:interval.last
+        depth = zeros(Int, length(range))
+        for record in eachoverlap(reader, interval)
+            if !BAM.ismapped(record) || !BAM.isprimary(record)
+                continue
+            end
+            aln = BAM.alignment(record)
+            for i in 1:BAM.seqlength(record)
+                j, op = seq2ref(aln, i)
+                if ismatchop(op) && j in range
+                    @inbounds depth[j - first(range) + 1] += 1
+                end
+            end
+        end
+        return depth
+    end
+end
+
+# Sequential computation.
+function transcript_depth0(bamfile, intervals)
+    reader = BAM.Reader(bamfile)
+    return map(intervals) do interval
+        return compute_depth(reader, interval)
+    end
+end
+
+# Parallel computation using pmap (open BAM.Reader inside the closure).
+function transcript_depth1(bamfile, intervals, batchsize)
+    pmap(intervals, batch_size=batchsize) do interval
+        reader = BAM.Reader(bamfile)
+        return compute_depth(reader, interval)
+    end
+end
+
+# Parallel computation using pmap (open BAM.Reader outside the closure).
+function transcript_depth2(bamfile, intervals, batchsize)
+    reader = BAM.Reader(bamfile)
+    return pmap(intervals, batch_size=batchsize) do interval
+        return compute_depth(reader, interval)
+    end
+end
+
+bamfile = expanduser("./data/SRR1238088.sort.bam")
+gff3file = expanduser("./data/TAIR10_GFF3_genes.gff")
+intervals = collect(Interval, Iterators.filter(r->GFF3.seqid(r)=="Chr1" && GFF3.featuretype(r)=="gene", GFF3.Reader(open(gff3file))))
+
+using DocOpt
+args = docopt("Usage: main.jl [--batch_size=<n>] <function>")
+batch_size = args["--batch_size"]
+if batch_size == nothing
+    batch_size = 30
+else
+    batch_size = parse(Int, batch_size)
+end
+f = eval(parse(args["<function>"]))
+func = () -> f == transcript_depth0 ? f(bamfile, intervals) : f(bamfile, intervals, batch_size)
+
+println(STDERR, sum(map(sum, func())))
+for i in 1:3
+    gc()
+    println(@elapsed func())
+end