diff --git a/src/buffered_input.jl b/src/buffered_input.jl
index dfa7b5a..ca1e3e9 100644
--- a/src/buffered_input.jl
+++ b/src/buffered_input.jl
@@ -15,40 +15,35 @@ mutable struct BufferedInput
     end
 end
 
-
-# Read and buffer n more characters
-function __fill(bi::BufferedInput, bi_input::IO, n::Integer)
-    for _ in 1:n
-        c = eof(bi_input) ? '\0' : read(bi_input, Char)
-        i = bi.offset + bi.avail + 1
+# Read and buffer `n` more characters
+function buffer!(bi::BufferedInput, n::Integer)::Nothing
+    for i in (bi.offset + bi.avail) .+ (1:n)
+        c = eof(bi.input) ? '\0' : read(bi.input, Char)
         if i ≤ length(bi.buffer)
             bi.buffer[i] = c
         else
             push!(bi.buffer, c)
         end
-        bi.avail += 1
     end
+    bi.avail += n
+    nothing
 end
 
-_fill(bi::BufferedInput, n::Integer) = __fill(bi, bi.input, n)
-
-# Peek the character in the i-th position relative to the current position.
-# (0-based)
-function peek(bi::BufferedInput, i::Integer=0)
-    i1 = i + 1
-    if bi.avail < i1
-        _fill(bi, i1 - bi.avail)
-    end
-    bi.buffer[bi.offset + i1]
+# Peek the character in the `i`-th position relative to the current position.
+function peek1(bi::BufferedInput, i::Integer=1)::Char
+    bi.avail < i && buffer!(bi, i - bi.avail)
+    bi.buffer[bi.offset + i]
 end
 
+# peek function for 0-based indices
+peek(bi::BufferedInput, i::Integer=0) = peek1(bi, i + 1)
 
 # Return the string formed from the first n characters from the current position
 # of the stream.
 function prefix(bi::BufferedInput, n::Integer=1)
     n1 = n + 1
     if bi.avail < n1
-        _fill(bi, n1 - bi.avail)
+        buffer!(bi, n1 - bi.avail)
     end
     String(bi.buffer[bi.offset .+ (1:n)])
 end