Expanding integer compositions

Question

Quick version: I would like Mathematica code that, for instance, turns {3,1,2} into {3,3,3,1,2,2}. More formally, given positive integers $c_1, \ldots, c_t$ which sum to $n$, produce the length $n$ list where each $c_i$ appears $c_i$ times in the original order.

Mathematical background: The list $\{c_1, \ldots, c_t\}$ is a composition of $n$, akin to an integer partition of $n$ where "order matters." The desired expansion is similar to the combinatorial representation of compositions using squares, dominos, generally $1 \times k$ blocks. So $\{3,1,2\}$ would be, left to right, a $1 \times 3$ block, a square, then a domino.

Motivation: Being able to get these representations in Mathematica would allow exploration of how much two compositions "agree" by counting the number of positions with the same numeral, e.g.,

$\{3,1,2\}$ ~ $\{3,3,3,1,2,2\}$ and $\{1,3,1,1\}$ ~ $\{1,3,3,3,1,1\}$

agree in two positions.

Answer 1

Flatten[ConstantArray[#,#] & /@ {3,1,2}]

{3, 3, 3, 1, 2, 2}

Answer 2

a = {3, 1, 2};

Using MapThread and Splice (new in 12.1)

MapThread[Splice @* Table, {a, a}]

{3, 3, 3, 1, 2, 2}

Answer 3

Using SequenceReplace:

a = {3, 1, 2};
SequenceReplace[a, {b_Integer} :> Splice@Table[b, b]]

For versions prior to v12.1

SequenceReplace[a, {b_Integer} :> Sequence @@ Table[b, b]]

---

{3, 3, 3, 1, 2, 2}