Chainlink threeway oracles

[QGarchery]

Handles the following cases:

• A/C, C/D and B/D are feeds (typically, WBTC/BTC, BTC/USD, USDC/USD).
• A/C and C/B are feeds (typically, WBTC/BTC and BTC/ETH).

Notice that it could also replace the simple oracle (instantiate the second base feed with the address 0)

Theory

Let's call four different tokens:

• $a_1$ with decimals $d_1$
• $a_2$ with decimals $d_2$
• $a_3$ with decimals $d_3$
• $b$ with decimals $d_b$.

Assume that we want an oracle for $a_1$ quoted in $b$. Assume that we have:

• a feed $f_1$ to go from $a_1$ to $a_2$, with decimals $o_1$
• a feed $f_2$ to go from $a_2$ to $a_3$, with decimals $o_2$
• a feed $g$ to go from $b$ to $a_3$, with decimals $o_g$

Let's write:

• $f_1 = p_1 * 10^{o_1 + d_1 - d_2}$, with $p_1$ denoting the price in real numbers of WEI of asset $a_1$ quoted in WEI of $a_2$
• $f_2 = p_2 * 10^{o_2 + d_2 - d_3}$, with $p_2$ denoting the price in real numbers of WEI of asset $a_2$ quoted in WEI of $a_3$
• $g = p_b * 10^{o_g + d_b - d_3}$, with $p_b$ denoting the price in real numbers of WEI of asset $b$ quoted in WEI of $a_3$

We want the result to be $$\frac{p_1 * p_2}{p_b} * 10^{36}$$
We can achieve that result with the following computation
$$\frac{f_1 * f_2 * SCALE}{g}$$
where $$SCALE = 10^{36 + (o_g + d_b) - (o_1 + o_2 + d_1)}$$