Rule for string->uint mapping and the storage position

[hievan2016]

Env

solc --version
solc, the solidity compiler commandline interface
Version: 0.4.7+commit.822622cf.Linux.g++

./geth version
Geth
Version: 1.4.7-stable-667a386d
Protocol Versions: [63 62 61]
Network Id: 1
Go Version: go1.5.4
OS: linux

Problem

We are trying to figure out the mapping rule and storage position of saved data
base on this formula

keccack(LeftPad32(key, 0), LeftPad32(map position, 0))

which is mentioned at

https://github.com/ethereum/wiki/wiki/JSON-RPC#eth_getstorageat

however, seems this works for uint->string mapping, but not for string->uint
mapping, could anyone pls help on this?

Below is the test contract

contract testMapping{
mapping(string=>uint) s2i; //position is 0
mapping(uint=>string) i2s; //position is 1

function setStr(string key) {
    s2i[key] = 100;            
}

function setInt(uint key) {
    i2s[key] = "zzz";
}

}

then we use the mapping to save the data by calling

setStr("aaa") // key is aaa (ascii : 616161)
setInt(200) // key is 200 (hex : c8)

the contract storage is

storage: {
345f7c6c888721344af4147de0834159e0b302300ba13c4e7b6c0b60d8f2314e: "a07a7a7a0000000000000000000000000000000000000000000000000000000006",
69e08a2904d77becc5ebdff9265102784e25436cf0dfbaeb5c932ed98ffc834d: "64"
}

the first entry shows "zzz" (ascii : 7a7a7a) is saved at position "345f7c6c888721344af4147de0834159e0b302300ba13c4e7b6c0b60d8f2314e",
this position could be got from the formula.

but when using "aaa" as the key to do string->uint mapping to save value 100 (0x64),
we cannot get the correct storage position "69e08a2904d77becc5ebdff9265102784e25436cf0dfbaeb5c932ed98ffc834d"
based on the formula, you can see, the sha3 value is incorrect.

var key = "0000000000000000000000000000000000000000000000000000000000616161" + "0000000000000000000000000000000000000000000000000000000000000000"
web3.sha3(key, {"encoding":"hex"})

"0x15b5515e45c94bd2fbc0aa103673f2c4899d2996fe1407ba1e6e5ef9ac66c2f5"

[chriseth]

Since strings are arbitrary length, you have to hash them first.

[hievan2016]

@chriseth

I still cannot get the correct hash value "69e08a2904d77becc5ebdff9265102784e25436cf0dfbaeb5c932ed98ffc834d", could you help check whether I miss anything?

var key0 = "00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000616161"
var key1 = web3.sha3(key0, {"encoding":"hex"})
var key2 = key1 + "0000000000000000000000000000000000000000000000000000000000000000"
web3.sha3(key2, {"encoding":"hex"})

sha3 : "0x8f34d90832af21f9fc31ef2888a966ad8090cdd9bc1a16ecf819059d118ae6db"

var key0 = "616161"
var key1 = web3.sha3(key0, {"encoding":"hex"})
var key2 = key1 + "0000000000000000000000000000000000000000000000000000000000000000"
web3.sha3(key2, {"encoding":"hex"})

sha3 : "0xe4998bf274557cf66b8123e4803a12c6dca7f047052b01d6b4b51776b0eb04d7"

var key0 = "0000000000000000000000000000000000000000000000000000000000616161"
var key1 = web3.sha3(key0, {"encoding":"hex"})
var key2 = key1 + "0000000000000000000000000000000000000000000000000000000000000000"
web3.sha3(key2, {"encoding":"hex"})

sha3 : "0x4b9ff950998e1bfecbf6ed1b5e7531fee7fac216130cafe3d3c03d0b1ba697b8"

[pirapira]

I would try the array access in the browser-solidity, and chase the stack & memory contents.

[hievan2016]

We are going to use getStorageAt() API, which requires the storage slot as parameter, so have to do sha3 hash to get the exact storage slot, so could you kindly help on this?

[hievan2016]

And, in fact, we need to use mapping in this way "mapping(string=>MyStruct)", and found the correct storage slot cannot be got, then simply the mapping to "mapping(string=>uint)" in this posting for easy reading/debugging.

[pirapira]

For your purpose, you can define

function getStr(string key) returns (uint) {
    return s2i[key];            
}

and call the function from web3.

[hievan2016]

@pirapira thanks for your reply, however, adding many get...() APIs is not what we want, we prefer using the calculated storage index and then call getStorageAt(), so let's continue figuring out how to calculate the storage slot.

@chriseth looking forwarding to your further input.

[chriseth]

Ah, I think your original version was fine except for padding the key. So I think the following should work:

var key = "616161"
var pos = "0000000000000000000000000000000000000000000000000000000000000000"
web3.sha3(key + pos, {"encoding":"hex"})

[hievan2016]

Awesome, it works, thanks a lot!

[ianmonkuk]

@hievan2016 @chriseth is there anyway to return an element of a mapping without knowing the key but using an index (i.e. pos 1, 2, etc)? Carrying on your example from above, if
i2s["aaa"] = 1
i2s["bbb"] = 2

Assuming we did not know the keys but we knew that i2s was in position 2?

I have worked out how to do this for arrays but not for maps.

Thanks Ian

[VoR0220]

@ianmonkuk you will have to create an iterated mapping. Search for it in the readthedocs page. You'll get the idea.

[chriseth]

@ianmonkuk please ask on http://gitter.im/ethereum/solidity or on http://ethereum.stackexchange.com

[ianmonkuk]

@VoR0220 thanks for responding. I should have clarified that I need to do this in the same way as discussed in this thread - using getStorageAt function which @hievan2016 was using. I am aware that this can be done relatively easily within the contract itself.

@chriseth there's been a lot of questions relate to this on stackexchange and they all point to the page below. This seems to suggest access to map elements (via getStorageAt) can only be accessed if the keys are known.

http://solidity.readthedocs.io/en/latest/miscellaneous.html#layout-of-state-variables-in-storage

Thanks, Ian