This is an iterative solution without built-ins. It uses the same indexing as the challenge spec.
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Let f be the function defined in the challenge spec and F the Fibonacci function defined as usual (i.e., with F(0) = 0). For a non-negative integer n, we have f(n) = F(n + 1). When 0 ≤ x < 1, the challenge spec defines f(n + x) as f(n) + (f(n + 1) - f(n))x.
Clearly, this just affects the base cases, but not the recursive formula, i.e., f(n) = f(n - 1) + f(n - 2) holds as it would for F. This means we can simplify the definition for non-integer arguments to the easier f(n) = f(n) + f(n - 1)x.
As others have noted in their answers, the recursive relationship also holds for non-integer arguments. This is easily verifiable, as
Since f(0) = f(1) = 1, f in constant in the interval [0, 1] and f(0 + x) = 1 for all x. Furthermore, f(-1) = F(0) = 0, so f(-1 + x) = f(-1) + (f(0) - f(-1))x = 0 + 1x = x. These base cases cover in [-1, 1), so together with the recursive formula, they complete the definition of f.
How it works
As before, let n + x be the only argument of our monadic program.
¡ is a quick, meaning that it consumes some links to its left and turns them into a quicklink.
¡ in particular consumes either one or two links.
<F:monad|dyad><N:any> calls the link N, returning r, and executes F a total of r times.
<nilad|missing><F:monad|dyad> sets r to the last command-line argument (or an input from STDIN in their absence) and executes F a total of r times.
1 is a nilad (a link without arguments), the second case applies, and
+¡ will execute
+ n times (a non-integer argument is rounded down). After each call to
+, the left argument of the quicklink is replaced with the return value, and the right argument with the previous value of the left argument.
As for the entire program,
Ḟ floors the input, yielding n; then
_ subtract the result from the input, yielding **x, which becomes the return value.
1+¡ then calls
+¡ – as described before – with left argument 1 = f(0 + x) and right argument x = f(-1 + x), which computes the desired output.