padic_radix_dot_strided, padic_radix_dot_strided_naive, padic_radix_dot_strided_delayed

Set res to $s \pm \sum_{i < len} a_i b_i$ with $a_i$ = vec1[i * stride1], $b_i$ = vec2[i * stride2], $s$ = initial (zero if initial is NULL) and the sign $-$ if subtract is nonzero. The naive version calls padic_radix_mul and padic_radix_add in a loop. The delayed version finds the minimum valuation and the precision of the result first, then accumulates the limb products of all terms in three-word accumulators, grouped by the residue of the digit offset modulo $e$ and by sign; at the end the residue classes are combined in one pass (each accumulator multiplied by $p^r$ for its residue $r$) and the carries are propagated once. The default uses the naive version for at most 4 terms at a precision of one limb, 3 terms at two limbs and 2 terms above (relative precision, or absolute if there is none), and, at precisions of up to two limbs, also for up to 7 terms whose valuations $v(a_i) + v(b_i)$ are not all equal.

When all terms are exact, the delayed version keeps the result exact if it fits in the context precision (as padic_radix_mul and padic_radix_add do), provided that the a priori extent of the terms, $\max_i (v(a_i) + e |a_i| + v(b_i) + e |b_i|)$ for units of $|a_i|$ and $|b_i|$ limbs, is at most one limb beyond the context precision; otherwise the sum is rounded to the context precision. In a context without finite precision, when the window of the delayed version (the extent of the exact terms, or the precision they determine) exceeds the size_limit of the context, it calls the naive version, which builds only the partial sums (and still gets exact cancellations of terms with huge valuations right).